SMTP

• Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined in RFC 821 (STD 15), and last updated by RFC 5321 (2008)[2] which includes the extended SMTP (ESMTP) additions, and is the protocol in widespread use today. SMTP is specified for outgoing mail transport and uses TCP port 25.
• While electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) to access their mail box accounts on a mail server.

 



The overall flow for message creation, mail transport and delivery may be illustrated as follows:

• sending MUA -> MSA -> sending MTA -> receiving MTA -> MDA -> Mailstore for retrieval by MUA
  • E-mail is submitted from a message user agent (MUA), the user’s email client, to a mail server (MSA), usually using SMTP. From there, the MSA delivers the mail to an MTA, often running on the same machine. A message may be directly submitted to an MTA: TCP port 587 is typically used for submission to MSAs (thence to MTAs), while TCP port 25 must be used for transferring to MTAs.
  • The MTA looks up the destination’s mail exchanger record in the DNS, and relays the mail to a server on record via TCP port 25 and SMTP. (The article on MX record discusses many factors in determining which server the sending MTA connects to.) Once the receiving MTA accepts the incoming message, it is delivered via a mail delivery agent (MDA) to a server which is designated for local mail delivery. The MDA either delivers the mail directly to storage, or forwards it over a network using either SMTP or the Local Mail Transfer Protocol (LMTP), a derivative of ESMTP 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, the email clients, using 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. Webmail clients may use either method, but the retrieval protocol is often not a formal standard. Some local mail servers and MUAs are capable of either push or pull mail retrieval.