What is port 110 used for?
Port 110 is POP3, a protocol mail programs use to collect messages from a mailbox on a server. The client connects, downloads the new mail and, by default, deletes it from the server.
POP3 was designed for a single computer that connects from time to time: mail ends up stored on that device. That makes it a poor fit for someone who reads mail on a phone and a laptop, since a message downloaded on one never appears on the other, which is the problem IMAP solved. On port 110 the user name, password and mail are sent in clear text unless the client upgrades the session with the STLS command.
Is port 110 TCP or UDP?
TCP. POP3 is a dialogue of commands and replies that must arrive complete and in order, so it runs over TCP port 110. IANA also lists UDP 110, but POP3 does not use it.
What is the secure version of POP3 on port 110?
POP3 over TLS on port 995, where the connection is encrypted from the first byte. Current guidance for mail clients (RFC 8314) is to use the TLS port in preference to clear-text 110.
Port 110 vs port 143: what is the difference?
Both retrieve mail. Port 110 is POP3, which downloads messages to one device. Port 143 is IMAP, which leaves mail on the server and keeps folders and read status in sync across several devices. Port 143 explained
Which CompTIA exams list port 110?
CompTIA lists port 110 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 “110, Post Office Protocol 3 (POP3)”.
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.
A user reads email on a laptop, then finds the same messages missing on their phone. The account is set up on port 110 on both. What explains it?
- POP3 downloaded the messages to the laptop and removed them from the server
- Port 110 is blocked on the phone
- The mailbox is full
- DNS is misconfigured
Show the answer
A. POP3 downloaded the messages to the laptop and removed them from the server. POP3's default behaviour is download and delete, so the second device finds nothing. IMAP on 143 or 993 keeps mail on the server for every device.
Sources: IANA Service Name and Transport Protocol Port Number Registry: port 110; RFC 1939: Post Office Protocol - Version 3; RFC 2595: Using TLS with IMAP, POP3 and ACAP; RFC 8314: Cleartext Considered Obsolete: Use of TLS for Email Submission and Access. The port number and how each exam lists it come from CompTIA's exam objectives, linked from the study plans above. The explanation and the question are CertSprint's own, checked against these sources on 12 November 2026.