Simple Mail Transfer Protocol (SMTP) | Application Layer Protocol | Web Technologies

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smtpSimple Mail Transfer Protocol (SMTP),
Web Technologies Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Simple Mail Transfer Protocol
SMTP is an internet standard for electronic mail transmission. Mail servers and other mail transfer agents use SMTP to send and receive mail messages. User level client mail application typically use SMTP for sending messages to a mail server for relaying. SMTP communication between mail servers use TCP port 25. Mail clients submit the outgoing email to mail server on port 587.

SMTP connection are secured by TLS, known as SMTPS can be made using START TLS. Although proprietary system and web mail system (gmail, yahoo) use their own non-standard protocol to access mail-box account on their own mail server, all use SMTP when sending or receiving email from outside their own system.

Email is submitted by a mail client (mail user agent, MUA) to a mail server (mail submission agent, MSA) using SMTP on TCP port 587/25. Mail submission agent delivers the mail to its mail transfer agent, MTA using SMTP.

The boundary MTA uses DNS to lookup the mail exchange record (MX record) for the recipient domain. MX record contains the name of target host. Based on target host and other factors, MTA selects an exchange server MTA connects to exchange server as SMTP client.

Message transfer can occur in a single connection between 2 MTAs or in a series of hops through intermediary systems. Once the final hop accepts the incoming message, it hands it to mail delivery agent (MDA) for local delivery. MDA saves the message in relevant mailbox format. An MDA may deliver messages directly to storage, or forward them over a network using SMTP or LMTP.

Once delivered to the local mail server, the mail is stored for batch retrieval by authentication mail client (MUAs). Mail is retrieved by end-user-application called email clients using IMAP or POP. SMTP defines message transport not message content.

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