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SESK: Stuff Everyone Should Know · sesk.info/aplus-smtp-port

Which port does SMTP use by default to send and relay email?

Port 25 is the default for SMTP.

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  1. 110
  2. 25 ✔
  3. 143

Why: SMTP (Simple Mail Transfer) is registered to port 25. POP3 uses 110 and IMAP uses 143, and both of those are for picking up mail, not sending it.

Huh, didn't know that: The IANA entry for port 25 is named 'Simple Mail Transfer'.

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References

Answer: 25. SMTP (Simple Mail Transfer) is registered to port 25. POP3 uses 110 and IMAP uses 143, and both of those are for picking up mail, not sending it.

  1. IANA Service Name and Transport Protocol Port Number Registry CSV www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.csv
    smtp,25,tcp,Simple Mail Transfer
    Checked 2026-10-11.

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