What uses port 110
POP3 downloads messages to one device and usually deletes them from the server, which doesn’t suit reading mail on several devices; IMAP (143/993) is the modern choice. Port 110 is unencrypted unless the client upgrades with STLS. Prefer POP3S on port 995 if you must use POP.
Should port 110 be open to the internet?
Restrict access. Only open this port to known networks or users, for example your office IP, a VPN or behind authentication.
Check what is listening on port 110
sudo ss -ltnp 'sport = :110'sudo lsof -nP -iTCP:110 -sTCP:LISTENGet-NetTCPConnection -LocalPort 110 -State Listen
netstat -ano | findstr :110No output means nothing is listening. If a service should be, check that it is running and which address it binds to: 127.0.0.1 only accepts local connections, 0.0.0.0 or :: accepts them on every interface.
Fix “address already in use” on port 110
Another process already holds the port. Find it with the commands above, then stop that service properly, or end the process:
sudo kill $(sudo lsof -t -iTCP:110 -sTCP:LISTEN)Stop-Process -Id (Get-NetTCPConnection -LocalPort 110 -State Listen).OwningProcessAllow port 110 from trusted networks only
The examples allow 110 only from 10.0.0.0/8; replace it with your own private network, office IP or VPN range.
sudo ufw allow from 10.0.0.0/8 to any port 110 proto tcp
# Close it again:
sudo ufw delete allow from 10.0.0.0/8 to any port 110 proto tcpsudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/8" port port="110" protocol="tcp" accept'
sudo firewall-cmd --reloadNew-NetFirewallRule -DisplayName "POP3 110/TCP" -Direction Inbound -Protocol TCP -LocalPort 110 -Action Allow -RemoteAddress 10.0.0.0/8On AWS, Google Cloud and Azure, also allow the port in the security group or network firewall rules; host firewalls and cloud firewalls are separate.
Test port 110 from another machine
nc -zv example.com 110Test-NetConnection example.com -Port 110Related ports
- Port 995 (POP3S): used by POP3 over TLS for securely downloading email.
- Port 143 (IMAP): used by IMAP for reading email that stays on the server.
- Port 993 (IMAPS): used by IMAP over TLS for securely reading email.
- Port 25 (SMTP): used by SMTP for delivering email between mail servers.
More email ports
465 (SMTPS) , 587 (SMTP submission) . See all ports in the port lookup.
Sources
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Frequently asked questions
What is port 110 used for?
Port 110 is used by POP3 for downloading email from a mail server. POP3 downloads messages to one device and usually deletes them from the server, which doesn’t suit reading mail on several devices; IMAP (143/993) is the modern choice. Port 110 is unencrypted unless the client upgrades with STLS. Prefer POP3S on port 995 if you must use POP.
Is it safe to open port 110?
Only open this port to known networks or users, for example your office IP, a VPN or behind authentication. Allow it only from the networks that need it, as in the firewall examples on this page.
How do I check if port 110 is open?
On the server, list listening sockets with "sudo ss -ltnp 'sport = :110'" on Linux or "sudo lsof -nP -i :110" on macOS. From another machine, run "nc -zv host 110" or, on Windows, "Test-NetConnection host -Port 110". Connection refused means nothing is listening; a timeout means a firewall is dropping the traffic.