Port 143: IMAP (Internet Message Access Protocol)

What is port 143 used for?

Port 143 is IMAP, the protocol mail programs use to read a mailbox that stays on the server. Folders, read and unread status and sent items are the same on every device that connects.

IMAP only reads and organises mail; sending is a separate job, done by SMTP on the submission port. On 143 the session begins in clear text, and a client has to issue STARTTLS before signing in to protect the password, a step that an attacker on the path can try to suppress. That is why current guidance prefers the dedicated TLS port. The A+ objectives list the port under the name Internet Mail Access Protocol; the standard's own name is Internet Message Access Protocol.

Is port 143 TCP or UDP?

TCP. IMAP runs over TCP port 143; IANA lists UDP 143 as reserved.

What is the secure version of IMAP on port 143?

IMAP over TLS on port 993, encrypted from the first byte. RFC 8314 recommends that mail clients use the implicit TLS port in preference to STARTTLS on 143.

Port 143 vs port 993: what is the difference?

Same protocol, different protection. Port 143 starts unencrypted and can be upgraded with STARTTLS. Port 993 wraps the whole IMAP session in TLS from the start, so no password can be sent before encryption is in place.

Which CompTIA exams list port 143?

CompTIA lists port 143 by number in the A+ objectives (Core 1); the Network+ port list does not include it.

• A+ Core 1 220-1201: objective 2.1 (Networking), listed as “143, Internet Mail Access Protocol (IMAP)”.

The quoted text is how the objectives document lists the port. The exam puts the port in a scenario, such as a firewall rule or a failed connection; it does not only ask for the number.

Check yourself: a practice question

Written by CertSprint for this page. It is not taken from any exam.