Port / TCP
110POP3
POP3 downloads messages from a server to a single client, traditionally deleting them from the mailbox afterwards.
If it is internet-facing: Port 110 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Behind a gateway
Transport
Encryption optional
Risk if public
Elevated
Exposure verdict
Port 110 should sit behind a reverse proxy, VPN, or source allow-list rather than being forwarded at the perimeter. Publishing it directly puts an administrative or internal interface on the open internet.
Encryption is available but negotiated or configured rather than guaranteed. A client that does not insist on it will silently fall back to plaintext.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other mail ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- Plain POP3 sends the mailbox password in the clear unless STARTTLS is both offered and required.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 110 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 110 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 110 -j DROPIf you do need it open to everyone
sudo ufw allow 110/tcpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -tlpn 'sport = :110'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iTCP:110 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 110 <host>A version probe confirms whether POP3 is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 110The quick yes/no when you only need to know whether a path exists through the firewall.
The safer path
port 995Move to port 995 — POP3S
POP3S does the same job with the session protected from the first byte, so there is no plaintext phase for an on-path attacker to read or strip. Migrating is usually a client configuration change plus a firewall swap.