Port 110 (POP3)

Post Office Protocol v3 for downloading mail, optionally upgraded with STLS.

Well-known (system) port, 0-1023 TCP Email Expose only to trusted sources

What port 110 is used for

Post Office Protocol v3 for downloading mail, optionally upgraded with STLS.

Expose only to trusted sources. Prefer POP3S on 995 or IMAP. If 110 is open, require STLS before login; plain POP3 logins leak passwords.

Service nameProtoDescriptionAssigneeReference

pop3 TCP Post Office Protocol - Version 3 Marshall Rose -

pop3 UDP Post Office Protocol - Version 3 Marshall Rose -

Linux: is anything listening on TCP 110?

sudo ss -tlnp 'sport = :110'

Linux / macOS: which process owns the port

sudo lsof -nP -i :110

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':110 '

Windows (Command Prompt)

netstat -ano | findstr :110

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 110 -State Listen | Select LocalAddress,OwningProcess

From another machine

nc -vz -w 5 HOST 110

From Windows

Test-NetConnection HOST -Port 110

Replace 203.0.113.10 with the address or network you trust. With ufw, make sure the default incoming policy is deny.

ufw (one trusted source)

sudo ufw allow from 203.0.113.10 to any port 110 proto tcp

iptables

sudo iptables -A INPUT -p tcp --dport 110 -s 203.0.113.10 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 110 -j DROP

nftables

sudo nft add rule inet filter input ip saddr 203.0.113.10 tcp dport 110 accept
sudo nft add rule inet filter input tcp dport 110 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'TCP 110 trusted' -Direction Inbound -Protocol TCP -LocalPort 110 -RemoteAddress 203.0.113.10 -Action Allow

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.