Glossary · Email

SMTP (Simple Mail Transfer Protocol)

SMTP is the standard protocol mail clients and servers use to send email and to relay it between servers, traditionally on TCP ports 25, 465 and 587.

Updated October 6, 2026

SMTP (Simple Mail Transfer Protocol) is the text-based protocol used to submit outgoing email and to pass it from one mail server to the next until it reaches the recipient's server. It only moves mail; reading it from a mailbox is done by IMAP or POP3.

Why it matters

Every email you send crosses at least one SMTP conversation, and most delivery problems are visible in it as numeric reply codes. Port 25 is used between servers, while port 587 (STARTTLS) and 465 (implicit TLS) are meant for clients submitting mail with authentication. Many cloud providers and home ISPs block outbound port 25 to limit spam, which is a common reason a self-hosted server cannot send.

Example

A simplified session:

S: 220 mx.example.com ESMTP ready
C: EHLO mail.sender.test
C: MAIL FROM:<news@sender.test>
S: 250 OK
C: RCPT TO:<jane@example.com>
S: 250 OK
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: (headers and body)
S: 250 Queued

A 5xx reply at any step is a permanent refusal and produces a bounce.

Common mistakes

  • Assuming SMTP is also used to read mail; it is not.
  • Sending without authentication or TLS on port 587, so credentials or content cross the network in clear text.
  • Testing only port 25 and concluding a server is down, when clients actually use 587 or 465.

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