Port 110 — POP3

Q: How do I check if port 110 is open?

Locally, sudo lsof -i :110 on macOS or Linux, or netstat -ano | findstr :110 on Windows. From outside, nc -zv host 110 tells you whether anything answers.

TCP well-known

What runs on port 110

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

POP3 downloads messages and, by default, deletes them from the server. That model predates having more than one device — read your mail on a laptop with POP3 and it is not on your phone. IMAP replaced it for that reason. Plain POP3 on 110 is unencrypted; 995 is the TLS version. If a mail client is configured for POP3 and messages keep disappearing from webmail, this is the explanation.

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.

Opening or blocking port 110

Finding the port is usually the first half of the job. Start narrow — the second rule is there for services that genuinely serve the public, and most do not:

# ufw — Debian, Ubuntu
sudo ufw allow from 10.0.0.0/8 to any port 110 proto tcp
sudo ufw allow 110/tcp                   # only if it genuinely serves the public

# firewalld — RHEL, Fedora, CentOS
sudo 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 --reload

# iptables, if you are still there
sudo iptables -A INPUT -p tcp --dport 110 -s 10.0.0.0/8 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 110 -j DROP

# Windows
New-NetFirewallRule -DisplayName "POP3 110" -Direction Inbound -Protocol TCP -LocalPort 110 -RemoteAddress 10.0.0.0/8 -Action Allow

10.0.0.0/8 stands in for a private range — substitute your own. Note that a firewall rule does not undo a service bound to 0.0.0.0; it only stops packets reaching it, and a misconfigured or disabled firewall then exposes it immediately.

Quick reference

Port	110
Protocol	TCP
Service	POP3
Range	Well-known (0–1023) — binding requires root on Unix

Frequently asked questions

What is port 110 used for?

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

Is it safe to open port 110?

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

How do I check if port 110 is open?

Locally, sudo lsof -i :110 on macOS or Linux, or netstat -ano | findstr :110 on Windows. From outside, nc -zv host 110 tells you whether anything answers.

Why do I get "address already in use" on port 110?

Another process is bound to it — often a previous run of your own program that did not exit cleanly. Find it with lsof -ti :110 and stop it, or configure your application to use a different port.

Can I change the port this service uses?

Almost always yes, in the service's configuration. Moving off a default port reduces automated scan noise, but it is obfuscation rather than security — a real attacker scans all 65,535.

Other common ports

All port numbers