Port 110 — POP3

TCP well-known

What runs on port 110

Mail retrieval that downloads and usually deletes messages from the server.

Security considerations

Unencrypted. Use port 995 (POP3S), or better, IMAP over TLS on 993.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :110
sudo ss -lntp | grep :110

# Windows
netstat -ano | findstr :110
Get-NetTCPConnection -LocalPort 110

# is it reachable from outside?
nc -zv example.com 110
curl -v telnet://example.com:110

Freeing the port

# find the process, then stop it
sudo lsof -ti :110 | xargs kill        # Linux / macOS
netstat -ano | findstr :110            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Should this port be open to the internet?

Only on a mail server. If this port is open on something that is not intentionally handling mail, you may be running an open relay — which will get the IP blacklisted quickly.

Quick reference

Frequently asked questions

Other common ports

• Port 20 — FTP data
• Port 21 — FTP control
• Port 22 — SSH / SFTP
• Port 23 — Telnet
• Port 25 — SMTP
• Port 53 — DNS
• Port 67 — DHCP server
• Port 68 — DHCP client
• Port 69 — TFTP
• Port 80 — HTTP
• Port 123 — NTP
• Port 143 — IMAP
• Port 161 — SNMP
• Port 389 — LDAP
• Port 443 — HTTPS
• Port 445 — SMB
• Port 465 — SMTPS
• Port 514 — Syslog

All port numbers