Port / TCP
587SMTP submission
The standard authenticated submission port that mail clients and applications use to hand outbound mail to their provider.
If it is internet-facing: Port 587 is routine where it is expected. Its risk lives in the service configuration behind it, not the port itself.
Exposure
Internet-facing
Transport
Encryption optional
Risk if public
Low
Exposure verdict
Port 587 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- Servers should require both STARTTLS and authentication here, and refuse to relay for unauthenticated senders.
- Use this rather than 25 for anything that is a client sending mail, not a server relaying it.
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 587/tcpiptables
sudo iptables -A INPUT -p tcp --dport 587 -j ACCEPTCheck what is really there
Listening locally
sudo ss -tlpn 'sport = :587'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:587 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 587 <host>A version probe confirms whether SMTP submission is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 587The quick yes/no when you only need to know whether a path exists through the firewall.