Glossary
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Port 465 (SMTPS)

TCP port 465 — SMTP over implicit TLS. Deprecated for years, then revived by RFC 8314 as an equal alternative to port 587 for mail submission.

Port 465 is the TCP port used for SMTPS — SMTP with implicit TLS. The TLS handshake starts immediately on connection, without any STARTTLS upgrade dance. IANA originally assigned it in 1997, deprecated it in 1998, and RFC 8314 (2018) revived it as an equal alternative to port 587 for mail submission.

SMTPS vs STARTTLS

  • Port 465 (SMTPS, implicit TLS) — TCP connect → TLS handshake immediately → SMTP conversation
  • Port 587 with STARTTLS — TCP connect → SMTP conversation in plaintext → STARTTLS command → TLS upgrade → resume SMTP

Both give you the same end result: authenticated, encrypted mail submission. Modern spec (RFC 8314) recommends port 465 with implicit TLS because implicit TLS is harder to strip via a downgrade attack.

Who Uses What

  • Gmail SMTP — port 465 or 587
  • Microsoft 365 SMTP — port 587 (port 465 not officially supported)
  • Amazon SES — both
  • Postmark — both
  • SendGrid — both
  • Zoho Mail SMTP — 465 or 587

Common Miss

  • Configuring a client for “SSL/TLS” on port 587 — port 587 uses STARTTLS, not implicit TLS; the connection fails
  • Configuring a client for “STARTTLS” on port 465 — port 465 goes straight to TLS; STARTTLS is a plaintext command that never fires

Match the port to the TLS mode: 465 = implicit TLS, 587 = STARTTLS.

Check whether port 465 is reachable with the port scanner.

Check the Port 25 glossary entry, the Port 587 entry, and read what an SPF record is — SPF authenticates the mail regardless of submission port.

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