Port 110 (POP3)
Port 110 is POP3, the protocol that downloads mail from the server to one device and usually deletes it there. Connections on 110 start unencrypted and may be upgraded with STLS.
- Service
- POP3
- Protocol
- TCP
- IANA registry
- Officially assigned: pop3 (Post Office Protocol - Version 3), TCP/UDP
Check if a port is open on your IP
We only check the address you are visiting from, one TCP port at a time. You cannot enter someone else’s address: this is not a port scanner.
Is it safe to open port 110?
RFC 8314 recommends implicit TLS on port 995 instead of plain POP3 on 110. On a mail server, close 110 to the internet unless old clients need it, and never allow password login without TLS.
Sources
“Port 110 is already in use”: how to fix it
The error means another program is already listening on port 110. Find its process ID (PID), then stop that program or start yours on a different port.
Windows (Command Prompt or PowerShell as administrator)
netstat -ano | findstr :110taskkill /PID <PID> /FmacOS
lsof -nP -iTCP:110 -sTCP:LISTENkill <PID>Linux
sudo ss -ltnp | grep :110sudo lsof -i :110sudo kill <PID>Replace <PID> with the number from the last column (Windows) or the PID column (macOS, Linux).
Port 110 vs 995
Same protocol, POP3. 110 starts unencrypted (optionally upgraded with STLS); 995 uses TLS from the start. Use 995.
Port 995 (POP3S)Why a port can look closed when your server is running
Most home networks sit behind a router doing NAT: the router owns the public address, and your computer has a private one such as 192.168.1.10. A connection from the internet reaches the router and stops there unless you add a port forwarding rule to your computer.
Many mobile and some home providers go further and use carrier-grade NAT (CGNAT): several customers share one public address, and your router itself gets an address from 100.64.0.0/10. Then no rule on your side can open a port; ask the provider for a public IP or use a tunnel or VPN service.
Even with forwarding in place, the firewall on the computer (Windows Defender Firewall, ufw, firewalld) must allow the port, and the program must listen on all interfaces (0.0.0.0), not only on 127.0.0.1.
UDP cannot be checked this way: a UDP service usually stays silent to packets it does not understand, so “no answer” means nothing. This check uses TCP only.
Frequently asked questions
What is port 110 used for?
Port 110 is POP3, the protocol that downloads mail from the server to one device and usually deletes it there. Connections on 110 start unencrypted and may be upgraded with STLS.
Is port 110 TCP or UDP?
Port 110 uses TCP. IANA reserves the number for both TCP and UDP, but in practice only TCP is used.
Is it safe to open port 110?
RFC 8314 recommends implicit TLS on port 995 instead of plain POP3 on 110. On a mail server, close 110 to the internet unless old clients need it, and never allow password login without TLS.
How do I free port 110 if it is already in use?
Find the process: on Windows run netstat -ano | findstr :110 and stop it with taskkill /PID <pid> /F; on macOS or Linux run lsof -i :110 and stop it with kill <pid>.