Port / TCP
25SMTP
SMTP moves mail between servers; it is the port an MX record points at, and the one mail relays listen on.
If it is internet-facing: Port 25 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Internet-facing
Transport
Encryption optional
Risk if public
Elevated
Exposure verdict
Port 25 normally answers from the internet, but the transport is not confidential by default. Enforce the encrypted variant before the traffic leaves your network.
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
2 notes- STARTTLS is negotiated after the greeting, so a downgrade attack can strip it unless MTA-STS or DANE is enforced.
- A relay that accepts mail for domains it does not host becomes an open relay and gets blocklisted quickly.
Recommended firewall rule
Allow it, and harden the service
This port is expected to answer from anywhere, so the firewall is not where its security comes from.
ufw
sudo ufw allow 25/tcpiptables
sudo iptables -A INPUT -p tcp --dport 25 -j ACCEPTCheck what is really there
Listening locally
sudo ss -tlpn 'sport = :25'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:25 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 25 <host>A version probe confirms whether SMTP is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 25The quick yes/no when you only need to know whether a path exists through the firewall.