Mail port numbers: smtp, submission, imap and pop3

By Liam CooperOctober 27, 2026IANA registry as updated October 26, 2026

IANA lists smtp on port 25, submission on 587 and submissions on 465 for sending mail, and imap on 143, imaps on 993, pop3 on 110 and pop3s on 995 for reading it. Five of those seven entries cite an RFC; smtp on 25 and pop3 on 110 cite none, although the 25 entry names an Internet-Draft, the revision of the SMTP specification.

The mail entries also differ in their references and transports. Port 465 carries three names, 143 cites a document that a newer one has since obsoleted, and the TLS entries for pop3 and imap were last modified in 2017 and 2021.

The mail entries

All of these sit in the System Port range. imap and imaps are assigned on TCP only, with the UDP numbers 143 and 993 held as reserved; pop3s still holds both transports. The registry last changed the imap pair on February 22, 2021, when RFC 9051 was added.

One transport or two

Five names, smtp, pop2, pop3, imap3 and submission, hold both TCP and UDP. imap and imaps are assigned on TCP only, and so is submissions, the only entry in the group with a registration date. pop3s on 995 is the exception among the TLS forms: it still holds a UDP number. None of these entries has an SCTP or DCCP assignment.

The entry for 25 has no RFC in its reference field, but it names an Internet-Draft, RFC-ietf-emailcore-rfc5321bis-43, the work on a revised SMTP specification. The registry last changed it on June 5, 2017.

The documents they cite

RFC metadata from the RFC Editor index shows how current each reference is.

RFC 3501, the IMAP4rev1 specification from 2003, has been obsoleted by RFC 9051 (August 2021). The registry entries for 143 and 993 cite both documents. RFC 8314 (January 2018) is the reference shared by 465, 993 and 995: its title names mail submission and access over TLS, and it is the reference on the 465 entry.

Port 465: three names on one number

Port 465 holds three entries. submissions, “Message Submission over TLS protocol”, is registered on TCP from December 12, 2017. urd, “URL Rendezvous Directory for SSM”, also holds TCP, and igmpv3lite holds UDP. That makes 465 one of only 12 numbers with three or more names. The pairing with 587 passes this site's counterpart rule: submissions is the TLS form of submission.

The older mail numbers

pop2 on 109 and imap3 on 220 (“Interactive Mail Access Protocol v3”) are still assigned on TCP and UDP, without dates or references. Port 24 is reserved in the registry for any private mail system; 26 has no assignment at all.

The other mail entries

Besides the seven numbers above, the registry holds a long tail of mail-related names from the early Internet. The table lists every entry whose label contains the word “mail”.

Only one of these 19 entries carries a registration date; the others predate the registry's date field. Several are vendor products, such as ccmail (“cc:mail/lotus”) on 3264 and the three Active Mail entries on 1396 to 1398. Several entries describe services of the early network: re-mail-ck on 50, “Remote Mail Checking Protocol”, csnet-ns on 105, “Mailbox Name Nameserver”, and biff on 512, the only UDP-only entry in the list, whose label is a full sentence about notifying users of new mail. imsp on 406 (“Interactive Mail Support Protocol”) and mupdate on 3905 (“Mailbox Update (MUPDATE) protocol”) are the closest relatives of IMAP in the list. Two other mail protocols appear under names without the word in their label: sieve on 4190 and qmtp on 209, “The Quick Mail Transfer Protocol”.

What the references say about status

Of the four documents cited by the main mail entries, RFC 6409 is an Internet Standard; the other three are Proposed Standards. RFC 6409 itself has been updated by RFC 8314, which in turn has one update, RFC 8997. RFC 3501 carries 13 updates in the index, more than any other RFC cited by a mail entry.

Searching for other mail names

19 registry labels contain the word “mail”, from mail notification (biff on 512 UDP) to vendor mail servers. A lookup for “mail” lists every one of them, and the ports 500–599 and ports 800–1023 pages show the submission and TLS numbers in context.