Editing Simple Mail Transfer Protocol (section)

==Mail processing model==
[[Image:SMTP-transfer-model.svg|thumb|300px|Blue arrows depict implementation of SMTP variations]]

Email is submitted by a mail client ([[mail user agent]], MUA) to a mail server ([[mail submission agent]], MSA) using SMTP on [[Transmission Control Protocol|TCP]] port 465 or 587. Most [[Mailbox Provider|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 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 [[Domain name system|DNS]] to look up the [[MX record|MX (mail exchanger) record]] for the recipient's domain (the part of the [[email address]] on the right of <code>@</code>). 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.

{{anchor|Hop}}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 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 {{IETF 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 the incoming message, it hands it to a [[mail delivery agent]] (MDA) for local delivery. An MDA saves messages in the relevant [[Email mailbox|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 [[Email forwarding|forward]] them over a network using SMTP or other protocol such as [[Local Mail Transfer Protocol]] (LMTP), 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 the traditional [[mbox]] mail file format or a proprietary system such as Microsoft Exchange/Outlook or [[Lotus Notes]]/[[IBM Lotus Domino|Domino]]. [[Webmail]] clients may use either method, but the retrieval protocol is often not a formal standard.

{{anchor|Internet Message Format}}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 the body of the message itself. STD 10 and {{IETF RFC|5321}} define SMTP (the envelope), while STD 11 and {{IETF RFC|5322}} define the message (header and body), formally referred to as the [[Internet Message Format]].