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Short note: SMTP

Short answer

SMTP is a text-based push protocol for sending email over TCP port 25, using commands HELO, MAIL FROM, RCPT TO, DATA, and QUIT. It uses store-and-forward with MX records and cannot retrieve mail; retrieval is done by POP3 or IMAP.

Write a short note on Simple Mail Transfer Protocol (SMTP).

SMTP (Simple Mail Transfer Protocol, RFC 5321) is the Internet standard application-layer protocol for sending email. It works over TCP, port 25 (587 for client submission, 465 for SMTP over TLS). It is a push protocol: the client pushes the mail to the server, which pushes it on to the next server until it reaches the recipient's mail server.

Features

  • Text-based, commands in 7-bit ASCII, replies with 3-digit codes (250 OK, 354 start mail input, 550 mailbox unavailable).
  • Persistent TCP connection, store-and-forward model.
  • Only transfers mail; retrieving uses POP3/IMAP.

Stages

  1. Connection setup: TCP handshake, server sends 220 greeting.
  2. Handshake: HELO/EHLO.
  3. Transfer: MAIL FROM, RCPT TO, DATA (header + body, ends with a line containing only ".").
  4. Close: QUIT.
Sender UA --> MTA (client) ===SMTP===> MTA (server) --> Mailbox

SMTP uses the receiving domain's MX record (found through DNS) to find the destination server. MIME is used to carry attachments and non-ASCII text.

✓ Answer checked by Claude Opus 5.5 (high effort)

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