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88-520: The contents of the string are as follows: and should be placed in the message body of an RFC 5322 compliant email message, without any line breaks or whitespace. Checksums for the string (68 bytes, no trailing newline) are as follows: There exist some varieties, notably the NAItube (which will carry a variable weight) and the GTphish (which will trigger specifically as a phishing mail), which are used in

176-511: A DNS name). This server will deliver outgoing messages on behalf of the user. Server administrators need to impose some control on which clients can use the server. This enables them to deal with abuse, for example spam . Two solutions have been in common use: Under this system, an ISP 's SMTP server will not allow access by users who are outside the ISP's network. More precisely, the server may only allow access to users with an IP address provided by

264-501: A Transmission Control Protocol (TCP) connection. An SMTP session consists of commands originated by an SMTP client (the initiating agent , sender, or transmitter) and corresponding responses from the SMTP server (the listening agent, or receiver) so that the session is opened, and session parameters are exchanged. A session may include zero or more SMTP transactions. An SMTP transaction consists of three command/reply sequences: Besides

352-447: A mail user agent (MUA), or a relay server's mail transfer agent (MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail. Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures. A MUA knows the outgoing mail SMTP server from its configuration. A relay server typically determines which server to connect to by looking up

440-541: A store-and-forward model. Email servers accept, forward, deliver, and store messages. Neither the users nor their computers are required to be online simultaneously; they need to connect, typically to a mail server or a webmail interface to send or receive messages or download it. Originally a text-only ASCII communications medium, Internet email was extended by MIME to carry text in expanded character sets and multimedia content such as images. International email , with internationalized email addresses using UTF-8 ,

528-447: A command is acknowledged by the server with a result code and response message (e.g., 250 Ok ). The transmission of the body of the mail message is initiated with a DATA command after which it is transmitted verbatim line by line and is terminated with an end-of-data sequence. This sequence consists of a new-line ( <CR><LF> ), a single full stop ( . ), followed by another new-line ( <CR><LF> ). Since

616-644: A consequence open mail relays have become rare, and many MTAs do not accept messages from open mail relays. The basic Internet message format used for email is defined by RFC   5322 , with encoding of non-ASCII data and multimedia content attachments defined in RFC 2045 through RFC 2049, collectively called Multipurpose Internet Mail Extensions or MIME . The extensions in International email apply only to email. RFC 5322 replaced RFC 2822 in 2008. Earlier, in 2001, RFC 2822 had in turn replaced RFC 822, which had been

704-555: A current Internet connection. The Post Office Protocol 3 (POP3) is a mail access protocol used by a client application to read messages from the mail server. Received messages are often deleted from the server . POP supports simple download-and-delete requirements for access to remote mailboxes (termed maildrop in the POP RFC's). POP3 allows downloading messages on a local computer and reading them even when offline. The Internet Message Access Protocol (IMAP) provides features to manage

792-474: A derivative of SMTP designed for this purpose. Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Mail is retrieved by end-user applications, called email clients, using Internet Message Access Protocol (IMAP), a protocol that both facilitates access to mail and manages stored mail, or the Post Office Protocol (POP) which typically uses

880-406: A domain name to an unqualified address. This behavior is helpful when the message being fixed is an initial submission, but dangerous and harmful when the message originated elsewhere and is being relayed. Cleanly separating mail into submission and relay was seen as a way to permit and encourage rewriting submissions while prohibiting rewriting relay. As spam became more prevalent, it was also seen as

968-413: A few characters outside that range and base64 for arbitrary binary data. The 8BITMIME and BINARY extensions were introduced to allow transmission of mail without the need for these encodings, but many mail transport agents may not support them. In some countries, e-mail software violates RFC   5322 by sending raw non-ASCII text and several encoding schemes co-exist; as a result, by default,

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1056-465: A file by email. Where larger files need to be shared, various file hosting services are available and commonly used. Simple Mail Transfer Protocol The Simple Mail Transfer Protocol ( SMTP ) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to

1144-523: A general structure for all existing and future extensions which aimed to add-in the features missing from the original SMTP. ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions. Message submission ( RFC   2476 ) and SMTP-AUTH ( RFC   2554 ) were introduced in 1998 and 1999, both describing new trends in email delivery. Originally, SMTP servers were typically internal to an organization, receiving mail for

1232-437: A large corporate environment, with a proprietary protocol specific to Novell Groupwise , Lotus Notes or Microsoft Exchange Servers . Programs used by users for retrieving, reading, and managing email are called mail user agents (MUAs). When opening an email, it is marked as "read", which typically visibly distinguishes it from "unread" messages on clients' user interfaces. Email clients may allow hiding read emails from

1320-718: A mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC   8314 . For retrieving messages, IMAP (which replaced the older POP3 ) is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync . SMTP's origins began in 1980, building on concepts implemented on the ARPANET since 1971. It has been updated, modified and extended multiple times. The protocol version in common use today has extensible structure with various extensions for authentication , encryption , binary data transfer, and internationalized email addresses . SMTP servers commonly use

1408-410: A mail store by programs called mail delivery agents (MDAs, also sometimes called local delivery agents, LDAs). Accepting a message obliges an MTA to deliver it, and when a message cannot be delivered, that MTA must send a bounce message back to the sender, indicating the problem. Users can retrieve their messages from servers using standard protocols such as POP or IMAP , or, as is more likely in

1496-402: A mailbox from multiple devices. Small portable devices like smartphones are increasingly used to check email while traveling and to make brief replies, larger devices with better keyboard access being used to reply at greater length. IMAP shows the headers of messages, the sender and the subject and the device needs to request to download specific messages. Usually, the mail is left in folders in

1584-404: A message body can contain a line with just a period as part of the text, the client sends two periods every time a line starts with a period; correspondingly, the server replaces every sequence of two periods at the beginning of a line with a single one. Such escaping method is called dot-stuffing . The server's positive reply to the end-of-data, as exemplified, implies that the server has taken

1672-697: A remote host to start processing of the mail queue on a server so it may receive messages destined to it by sending a corresponding command. The original TURN command was deemed insecure and was extended in RFC   1985 with the ETRN command which operates more securely using an authentication method based on Domain Name System information. An email client needs to know the IP address of its initial SMTP server and this has to be given as part of its configuration (usually given as

1760-433: A remote server on demand, SMTP has a feature to initiate mail queue processing on a remote server (see Remote Message Queue Starting below). POP and IMAP are unsuitable protocols for relaying mail by intermittently-connected machines; they are designed to operate after final delivery, when information critical to the correct operation of mail relay (the "mail envelope") has been removed. Remote Message Queue Starting enables

1848-457: A single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a smart host . Each process is an MTA (an SMTP server) in its own right. The boundary MTA uses DNS to look up the MX (mail exchanger) record for

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1936-693: A smartphone ranges and differs dramatically across different countries. For example, in comparison to 75% of those consumers in the US who used it, only 17% in India did. As of 2010 , the number of Americans visiting email web sites had fallen 6 percent after peaking in November 2009. For persons 12 to 17, the number was down 18 percent. Young people preferred instant messaging , texting and social media . Technology writer Matt Richtel said in The New York Times that email

2024-502: A user is mobile, and may use different ISPs to connect to the internet, this kind of usage restriction is onerous, and altering the configured outbound email SMTP server address is impractical. It is highly desirable to be able to use email client configuration information that does not need to change. Modern SMTP servers typically require authentication of clients by credentials before allowing access, rather than restricting access by location as described earlier. This more flexible system

2112-587: A way to provide authorization for mail being sent out from an organization, as well as traceability. This separation of relay and submission quickly became a foundation for modern email security practices. As this protocol started out purely ASCII text-based, it did not deal well with binary files, or characters in many non-English languages. Standards such as Multipurpose Internet Mail Extensions ( MIME ) were developed to encode binary files for transfer through SMTP. Mail transfer agents (MTAs) developed after Sendmail also tended to be implemented 8-bit clean , so that

2200-458: Is a ubiquitous and very widely used communication medium; in current use, an email address is often treated as a basic and necessary part of many processes in business, commerce, government, education, entertainment, and other spheres of daily life in most countries. Email operates across computer networks , primarily the Internet , and also local area networks . Today's email systems are based on

2288-588: Is also part of the header, as defined below. SMTP defines the trace information of a message saved in the header using the following two fields: Other fields added on top of the header by the receiving server may be called trace fields . Internet email was designed for 7-bit ASCII. Most email software is 8-bit clean , but must assume it will communicate with 7-bit servers and mail readers. The MIME standard introduced character set specifiers and two content transfer encodings to enable transmission of non-ASCII data: quoted printable for mostly 7-bit content with

2376-499: Is available). The client notifies the receiver of the originating email address of the message in a MAIL FROM command. This is also the return or bounce address in case the message cannot be delivered. In this example the email message is sent to two mailboxes on the same SMTP server: one for each recipient listed in the To: and Cc: header fields. The corresponding SMTP command is RCPT TO . Each successful reception and execution of

2464-760: Is friendly to mobile users and allows them to have a fixed choice of configured outbound SMTP server. SMTP Authentication , often abbreviated SMTP AUTH, is an extension of the SMTP in order to log in using an authentication mechanism. Communication between mail servers generally uses the standard TCP port 25 designated for SMTP. Mail clients however generally don't use this, instead using specific "submission" ports. Mail services generally accept email submission from clients on one of: Port 2525 and others may be used by some individual providers, but have never been officially supported. Many Internet service providers now block all outgoing port 25 traffic from their customers. Mainly as an anti-spam measure, but also to cure for

2552-461: Is needed for most non-text data and some text formats). In 2012, the SMTPUTF8 extension was created to support UTF-8 text, allowing international content and addresses in non- Latin scripts like Cyrillic or Chinese . Many people contributed to the core SMTP specifications, among them Jon Postel , Eric Allman , Dave Crocker, Ned Freed , Randall Gellens, John Klensin , and Keith Moore . Email

2640-459: Is no technical restriction on the size or number of attachments. However, in practice, email clients, servers , and Internet service providers implement various limitations on the size of files, or complete email – typically to 25MB or less. Furthermore, due to technical reasons, attachment sizes as seen by these transport systems can differ from what the user sees, which can be confusing to senders when trying to assess whether they can safely send

2728-494: Is often successfully used to send special sales offerings and new product information. Depending on the recipient's culture, email sent without permission—such as an "opt-in"—is likely to be viewed as unwelcome " email spam ". Many users access their personal emails from friends and family members using a personal computer in their house or apartment. Email has become used on smartphones and on all types of computers. Mobile "apps" for email increase accessibility to

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2816-471: Is standardized but not widely adopted. The term electronic mail has been in use with its modern meaning since 1975, and variations of the shorter E-mail have been in use since 1979: The service is often simply referred to as mail , and a single piece of electronic mail is called a message . The conventions for fields within emails—the "To", "From", "CC", "BCC" etc.—began with RFC-680 in 1975. An Internet email consists of an envelope and content ;

2904-417: Is submitted by a mail client ( mail user agent , MUA) to a mail server ( mail submission agent , MSA) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. The MSA delivers the mail to its mail transfer agent (MTA). Often, these two agents are instances of the same software launched with different options on the same machine. Local processing can be done either on

2992-419: Is supplied separately to the transport protocol, SMTP , which may be extracted from the header content. The "To:" field is similar to the addressing at the top of a conventional letter delivered according to the address on the outer envelope. In the same way, the "From:" field may not be the sender. Some mail servers apply email authentication systems to messages relayed. Data pertaining to the server's activity

3080-655: Is that connecting to an MSA requires SMTP Authentication . SMTP is a delivery protocol only. In normal use, mail is "pushed" to a destination mail server (or next-hop mail server) as it arrives. Mail is routed based on the destination server, not the individual user(s) to which it is addressed. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for use by individual users retrieving messages and managing mailboxes . To permit an intermittently-connected mail server to pull messages from

3168-435: The MX (Mail eXchange) DNS resource record for each recipient's domain name . If no MX record is found, a conformant relaying server (not all are) instead looks up the A record . Relay servers can also be configured to use a smart host . A relay server initiates a TCP connection to the server on the " well-known port " for SMTP: port 25, or for connecting to an MSA, port 587. The main difference between an MTA and an MSA

3256-479: The McAfee implementation of SpamAssassin . This malware -related article is a stub . You can help Misplaced Pages by expanding it . Email Email (short for electronic mail ; alternatively spelled e-mail ) is a method of transmitting and receiving messages using electronic devices. It was conceived in the late–20th century as the digital version of, or counterpart to, mail (hence e- + mail ). Email

3344-576: The Transmission Control Protocol on port number 25 (between servers) and 587 (for submission from authenticated clients), both with or without encryption. Various forms of one-to-one electronic messaging were used in the 1960s. Users communicated using systems developed for specific mainframe computers . As more computers were interconnected, especially in the U.S. Government's ARPANET , standards were developed to permit exchange of messages between different operating systems. Mail on

3432-702: The ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented, but is discussed in RFC   196 ; and the SNDMSG program, which Ray Tomlinson of BBN adapted that year to send messages across two computers on the ARPANET. A further proposal for a Mail Protocol was made in RFC 524 in June 1973, which was not implemented. The use of the File Transfer Protocol (FTP) for "network mail" on

3520-710: The ARPANET was proposed in RFC 469 in March 1973. Through RFC 561, RFC 680, RFC 724, and finally RFC 733 in November 1977, a standardized framework for "electronic mail" using FTP mail servers on was developed. SMTP grew out of these standards developed during the 1970s. Ray Tomlinson discussed network mail among the International Network Working Group in INWG Protocol note 2 , written in September 1974. INWG discussed protocols for electronic mail in 1979, which

3608-459: The ISP, which is equivalent to requiring that they are connected to the Internet using that same ISP. A mobile user may often be on a network other than that of their normal ISP, and will then find that sending email fails because the configured SMTP server choice is no longer accessible. This system has several variations. For example, an organisation's SMTP server may only provide service to users on

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3696-504: The URL in the To: field. Many clients also support query string parameters for the other email fields, such as its subject line or carbon copy recipients. Many email providers have a web-based email client. This allows users to log into the email account by using any compatible web browser to send and receive their email. Mail is typically not downloaded to the web client, so it cannot be read without

3784-432: The ability to be used for more frequent communication between users and allowed them to check their email and write messages throughout the day. As of 2011 , there were approximately 1.4 billion email users worldwide and 50 billion non-spam emails that were sent daily. Individuals often check emails on smartphones for both personal and work-related messages. It was found that US adults check their email more than they browse

3872-417: The ability to include in-line links and images, set apart previous messages in block quotes , wrap naturally on any display, use emphasis such as underlines and italics , and change font styles. Disadvantages include the increased size of the email, privacy concerns about web bugs , abuse of HTML email as a vector for phishing attacks and the spread of malicious software . Some e-mail clients interpret

3960-523: The alternate "just send eight" strategy could be used to transmit arbitrary text data (in any 8-bit ASCII-like character encoding) via SMTP. Mojibake was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only ASCII . 8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting some binary files to be transmitted almost as easily as plain text (limits on line length and permitted octet values still apply, so that MIME encoding

4048-447: The attachments. Others separate attachments from messages and save them in a specific directory. The URI scheme , as registered with the IANA, defines the mailto: scheme for SMTP email addresses. Though its use is not strictly defined, URLs of this form are intended to be used to open the new message window of the user's mail client when the URL is activated, with the address as defined by

4136-466: The body as HTML even in the absence of a Content-Type: html header field; this may cause various problems. Some web-based mailing lists recommend all posts be made in plain text, with 72 or 80 characters per line for all the above reasons, and because they have a significant number of readers using text-based email clients such as Mutt . Various informal conventions evolved for marking up plain text in email and usenet posts, which later led to

4224-512: The body of the message itself. STD 10 and RFC   5321 define SMTP (the envelope), while STD 11 and RFC   5322 define the message (header and body), formally referred to as the Internet Message Format . SMTP is a connection-oriented , text-based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically

4312-414: The content consists of a header and a body . Computer-based messaging between users of the same system became possible after the advent of time-sharing in the early 1960s, with a notable implementation by MIT 's CTSS project in 1965. Most developers of early mainframes and minicomputers developed similar, but generally incompatible, mail applications. In 1971 the first ARPANET network mail

4400-523: The corporate SMTP server.) This issue, a consequence of the rapid expansion and popularity of the World Wide Web , meant that SMTP had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited email ( spam ). Work on message submission ( RFC   2476 ) was originally started because popular mail servers would often rewrite mail in an attempt to fix problems in it, for example, adding

4488-463: The current destination(s) had been queued. The information that the client sends in the HELO and MAIL FROM commands are added (not seen in example code) as additional header fields to the message by the receiving server. It adds a Received and Return-Path header field, respectively. Some clients are implemented to close the connection after the message is accepted ( 250 Ok: queued as 12345 ), so

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4576-441: The developed world, and it is one of the key parts of an 'e-revolution' in workplace communication (with the other key plank being widespread adoption of highspeed Internet ). A sponsored 2010 study on workplace communication found 83% of U.S. knowledge workers felt email was critical to their success and productivity at work. It has some key benefits to business and other organizations, including: Email marketing via " opt-in "

4664-560: The development of formal languages like setext (c. 1992) and many others , the most popular of them being markdown . Some Microsoft email clients may allow rich formatting using their proprietary Rich Text Format (RTF), but this should be avoided unless the recipient is guaranteed to have a compatible email client. Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software programs called mail transfer agents (MTAs); and delivered to

4752-448: The exchange.) After the message sender (SMTP client) establishes a reliable communications channel to the message receiver (SMTP server), the session is opened with a greeting by the server, usually containing its fully qualified domain name (FQDN), in this case smtp.example.com . The client initiates its dialog by responding with a HELO command identifying itself in the command's parameter with its FQDN (or an address literal if none

4840-421: The file system. Some clients save individual messages as separate files, while others use various database formats, often proprietary, for collective storage. A historical standard of storage is the mbox format. The specific format used is often indicated by special filename extensions : Some applications (like Apple Mail ) leave attachments encoded in messages for searching while also saving separate copies of

4928-399: The final restrictions on carrying commercial traffic over the Internet ended in 1995, a combination of factors made the current Internet suite of SMTP, POP3 and IMAP email protocols the standard (see Protocol Wars ). The following is a typical sequence of events that takes place when sender Alice transmits a message using a mail user agent (MUA) addressed to the email address of

5016-450: The following fields: RFC 3864 describes registration procedures for message header fields at the IANA ; it provides for permanent and provisional field names, including also fields defined for MIME, netnews, and HTTP, and referencing relevant RFCs. Common header fields for email include: The To: field may be unrelated to the addresses to which the message is delivered. The delivery list

5104-560: The former began in the late 1970s and the latter became the world's largest selling email system. The Simple Mail Transfer Protocol (SMTP) was implemented on the ARPANET in 1983. LAN email systems emerged in the mid-1980s. For a time in the late 1980s and early 1990s, it seemed likely that either a proprietary commercial system or the X.400 email system, part of the Government Open Systems Interconnection Profile (GOSIP), would predominate. However, once

5192-452: The higher cost they have when leaving it open, perhaps by charging more from the few customers that require it open. A typical example of sending a message via SMTP to two mailboxes ( alice and theboss ) located in the same mail domain ( example.com ) is reproduced in the following session exchange. (In this example, the conversation parts are prefixed with S: and C: , for server and client , respectively; these labels are not part of

5280-509: The inbox so the user can focus on the unread. Mail can be stored on the client , on the server side, or in both places. Standard formats for mailboxes include Maildir and mbox . Several prominent email clients use their own proprietary format and require conversion software to transfer email between them. Server-side storage is often in a proprietary format but since access is through a standard protocol such as IMAP, moving email from one server to another can be done with any MUA supporting

5368-508: The incoming message, it hands it to a mail delivery agent (MDA) for local delivery. An MDA saves messages in the relevant mailbox format. As with sending, this reception can be done using one or multiple computers, but in the diagram above the MDA is depicted as one box near the mail exchanger box. An MDA may deliver messages directly to storage, or forward them over a network using SMTP or other protocol such as Local Mail Transfer Protocol (LMTP),

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5456-484: The intermediate reply for DATA, each server's reply can be either positive (2xx reply codes) or negative. Negative replies can be permanent (5xx codes) or transient (4xx codes). A reject is a permanent failure and the client should send a bounce message to the server it received it from. A drop is a positive response followed by message discard rather than delivery. The initiating host, the SMTP client, can be either an end-user's email client , functionally identified as

5544-503: The last two lines may actually be omitted. This causes an error on the server when trying to send the 221 Bye reply. Clients learn a server's supported options by using the EHLO greeting, as exemplified below, instead of the original HELO . Clients fall back to HELO only if the server does not support EHLO greeting. Modern clients may use the ESMTP extension keyword SIZE to query

5632-655: The limit is 998 characters. Header fields defined by RFC 5322 contain only US-ASCII characters; for encoding characters in other sets, a syntax specified in RFC 2047 may be used. In some examples, the IETF EAI working group defines some standards track extensions, replacing previous experimental extensions so UTF-8 encoded Unicode characters may be used within the header. In particular, this allows email addresses to use non-ASCII characters. Such addresses are supported by Google and Microsoft products, and promoted by some government agents. The message header must include at least

5720-515: The mail server. Messaging Application Programming Interface (MAPI) is used by Microsoft Outlook to communicate to Microsoft Exchange Server —and to a range of other email server products such as Axigen Mail Server , Kerio Connect , Scalix , Zimbra , HP OpenMail , IBM Lotus Notes , Zarafa , and Bynari where vendors have added MAPI support to allow their products to be accessed directly via Outlook. Email has been widely accepted by businesses, governments and non-governmental organizations in

5808-402: The medium for users who are out of their homes. While in the earliest years of email, users could only access email on desktop computers, in the 2010s, it is possible for users to check their email when they are away from home, whether they are across town or across the world. Alerts can also be sent to the smartphone or other devices to notify them immediately of new messages. This has given email

5896-510: The message in a non-Latin alphabet language appears in non-readable form (the only exception is a coincidence if the sender and receiver use the same encoding scheme). Therefore, for international character sets , Unicode is growing in popularity. Most modern graphic email clients allow the use of either plain text or HTML for the message body at the option of the user. HTML email messages often include an automatic-generated plain text copy for compatibility. Advantages of HTML include

5984-496: The message that they will try to deliver. The probability that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes. The limiting timeout is specified to be 10 minutes. The QUIT command ends the session. If the email has other recipients located elsewhere, the client would QUIT and connect to an appropriate SMTP server for subsequent recipients after

6072-426: The message, as unstructured text, sometimes containing a signature block at the end. The header is separated from the body by a blank line. RFC 5322 specifies the syntax of the email header. Each email message has a header (the "header section" of the message, according to the specification), comprising a number of fields ("header fields"). Each field has a name ("field name" or "header field name"), followed by

6160-459: The most popular operating system on the Internet, Sendmail became the most common MTA (mail transfer agent). The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial man-in-the-middle attack , spoofing , and spamming , and requiring any binary data to be encoded to readable text before transmission. Due to absence of a proper authentication mechanism, by design every SMTP server

6248-443: The network all the time. Both used a store and forward mechanism and are examples of push technology . Though Usenet's newsgroups were still propagated with UUCP between servers, UUCP as a mail transport has virtually disappeared along with the " bang paths " it used as message routing headers. Sendmail , released with 4.1cBSD in 1983, was one of the first mail transfer agents to implement SMTP. Over time, as BSD Unix became

6336-403: The organization from the outside , and relaying messages from the organization to the outside . But as time went on, SMTP servers (mail transfer agents), in practice, were expanding their roles to become message submission agents for mail user agents , some of which were now relaying mail from the outside of an organization. (e.g. a company executive wishes to send email while on a trip using

6424-450: The protocol. Many current email users do not run MTA, MDA or MUA programs themselves, but use a web-based email platform, such as Gmail or Yahoo! Mail , that performs the same tasks. Such webmail interfaces allow users to access their mail with any standard web browser , from any computer, rather than relying on a local email client. Upon reception of email messages, email client applications save messages in operating system files in

6512-422: The recipient's domain (the part of the email address on the right of @ ). The MX record contains the name of the target MTA. Based on the target host and other factors, the sending MTA selects a recipient server and connects to it to complete the mail exchange. Message transfer can occur in a single connection between two MTAs, or in a series of hops through intermediary systems. A receiving SMTP server may be

6600-454: The recipient. In addition to this example, alternatives and complications exist in the email system: Many MTAs used to accept messages for any recipient on the Internet and do their best to deliver them. Such MTAs are called open mail relays . This was very important in the early days of the Internet when network connections were unreliable. However, this mechanism proved to be exploitable by originators of unsolicited bulk email and as

6688-436: The responsibility of delivering the message. A message can be doubled if there is a communication failure at this time, e.g. due to a power outage: Until the sender has received that 250 Ok reply, it must assume the message was not delivered. On the other hand, after the receiver has decided to accept the message, it must assume the message has been delivered to it. Thus, during this time span, both agents have active copies of

6776-444: The same network, enforcing this by firewalling to block access by users on the wider Internet. Or the server may perform range checks on the client's IP address. These methods were typically used by corporations and institutions such as universities which provided an SMTP server for outbound mail only for use internally within the organisation. However, most of these bodies now use client authentication methods, as described below. Where

6864-705: The separator character ":", and a value ("field body" or "header field body"). Each field name begins in the first character of a new line in the header section, and begins with a non- whitespace printable character . It ends with the separator character ":". The separator is followed by the field value (the "field body"). The value can continue onto subsequent lines if those lines have space or tab as their first character. Field names and, without SMTPUTF8 , field bodies are restricted to 7-bit ASCII characters. Some non-ASCII values may be represented using MIME encoded words . Email header fields can be multi-line, with each line recommended to be no more than 78 characters, although

6952-470: The server for the maximum message size that will be accepted. Older clients and servers may try to transfer excessively sized messages that will be rejected after consuming network resources, including connect time to network links that is paid by the minute. Users can manually determine in advance the maximum size accepted by ESMTP servers. The client replaces the HELO command with the EHLO command. Thus smtp2.example.com declares that it can accept

7040-506: The standard for Internet email for decades. Published in 1982, RFC 822 was based on the earlier RFC 733 for the ARPANET. Internet email messages consist of two sections, "header" and "body". These are known as "content". The header is structured into fields such as From, To, CC, Subject, Date, and other information about the email. In the process of transporting email messages between systems, SMTP communicates delivery parameters and information using message header fields. The body contains

7128-429: The traditional mbox mail file format or a proprietary system such as Microsoft Exchange/Outlook or Lotus Notes / Domino . Webmail clients may use either method, but the retrieval protocol is often not a formal standard. SMTP defines message transport , not the message content . Thus, it defines the mail envelope and its parameters, such as the envelope sender , but not the header (except trace information ) nor

7216-418: The ultimate destination, an intermediate "relay" (that is, it stores and forwards the message) or a "gateway" (that is, it may forward the message using some protocol other than SMTP). Per RFC   5321 section 2.1, each hop is a formal handoff of responsibility for the message, whereby the receiving server must either deliver the message or properly report the failure to do so. Once the final hop accepts

7304-434: The web or check their Facebook accounts, making email the most popular activity for users to do on their smartphones. 78% of the respondents in the study revealed that they check their email on their phone. It was also found that 30% of consumers use only their smartphone to check their email, and 91% were likely to check their email at least once per day on their smartphone. However, the percentage of consumers using email on

7392-415: Was an open mail relay . The Internet Mail Consortium (IMC) reported that 55% of mail servers were open relays in 1998, but less than 1% in 2002. Because of spam concerns most email providers blocklist open relays, making original SMTP essentially impractical for general use on the Internet. In November 1995, RFC   1869 defined Extended Simple Mail Transfer Protocol (ESMTP), which established

7480-480: Was developed around the same time as Usenet , a one-to-many communication network with some similarities. SMTP became widely used in the early 1980s. At the time, it was a complement to the Unix to Unix Copy Program (UUCP), which was better suited for handling email transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to

7568-551: Was like the VCR , vinyl records and film cameras —no longer cool and something older people do. A 2015 survey of Android users showed that persons 13 to 24 used messaging apps 3.5 times as much as those over 45, and were far less likely to use email. Email messages may have one or more attachments, which are additional files that are appended to the email. Typical attachments include Microsoft Word documents, PDF documents, and scanned images of paper documents. In principle, there

7656-789: Was referenced by Jon Postel in his early work on Internet email. Postel first proposed an Internet Message Protocol in 1979 as part of the Internet Experiment Note (IEN) series. In 1980, Postel and Suzanne Sluizer published RFC   772 which proposed the Mail Transfer Protocol as a replacement for the use of the FTP for mail. RFC   780 of May 1981 removed all references to FTP and allocated port 57 for TCP and UDP , an allocation that has since been removed by IANA . In November 1981, Postel published RFC   788 "Simple Mail Transfer Protocol". The SMTP standard

7744-570: Was sent, introducing the now-familiar address syntax with the ' @ ' symbol designating the user's system address. Over a series of RFCs , conventions were refined for sending mail messages over the File Transfer Protocol . Proprietary electronic mail systems soon began to emerge. IBM , CompuServe and Xerox used in-house mail systems in the 1970s; CompuServe sold a commercial intraoffice mail product in 1978 to IBM and to Xerox from 1981. DEC's ALL-IN-1 and Hewlett-Packard's HPMAIL (later HP DeskManager) were released in 1982; development work on

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