Port 110 POP3
Port 110 is the IANA-assigned pop3 port for Post Office Protocol - Version 3 (TCP / UDP).
What is port 110 used for?
Port 110 is assigned to pop3 and uses TCP / UDP. The IANA registry describes the service as “Post Office Protocol - Version 3”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach pop3 connects to port 110. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 110 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [Marshall_Rose].
Check and block port 110
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :110'
sudo ss -ulpn 'sport = :110'Check on Windows
Get-NetTCPConnection -LocalPort 110 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 110 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 110/tcp
sudo ufw deny 110/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 110 -j DROP
sudo iptables -A INPUT -p udp --dport 110 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 110 safe to expose?
POP3 sends credentials and mail in clear text unless wrapped in TLS. Prefer POP3S on 995 and block cleartext mailbox access from untrusted networks. Expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Where port 110 is usually used
Most traffic on port 110 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Mail clients sending messages through a relay or fetching them from a mailbox.
- Mail servers exchanging messages for a domain or syncing folders with clients.
- Deliverability, authentication and TLS tests that connect directly to the mail service.
Test the POP3 listener
The TLS handshake reveals the certificate and proves the mail service is reachable.
openssl s_client -connect mail.example.com:110 -starttls pop3Common problems with port 110
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 110. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :110' or sudo lsof -i :110. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 110 at a glance
| Port number | 110 |
|---|---|
| Service name | pop3 |
| Transport protocol | TCP / UDP |
| Category | |
| Range | Well-known port (0–1023) |
| Risk level | Medium |
| Registered assignee | [Marshall_Rose] |
| Registry description | Post Office Protocol - Version 3 |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 110 used for?
Port 110 (POP3) uses TCP / UDP. Post Office Protocol - Version 3.
How do I check if port 110 is open?
On Linux run `sudo ss -tlpn 'sport = :110'` and `sudo ss -ulpn 'sport = :110'`. On Windows run `Get-NetTCPConnection -LocalPort 110 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 110 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 110; empty output usually means nothing is bound to it.
How do I block port 110?
With UFW run `sudo ufw deny 110/tcp` and `sudo ufw deny 110/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 110 -j DROP` and `sudo iptables -A INPUT -p udp --dport 110 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 110 safe to expose?
POP3 sends credentials and mail in clear text unless wrapped in TLS. Prefer POP3S on 995 and block cleartext mailbox access from untrusted networks. Nevertheless, expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy