Port 25 (SMTP)
Mail transfer between mail servers.
- Transport
- TCP
- Internet exposure
- Public by design
Normally public, but keep the software patched and hardened.
Security notes
- Only mail servers should send on port 25. Workstations sending outbound SMTP often indicate spam malware.
- An open relay lets anyone send mail through your server and gets it blocklisted.
- Client mail submission belongs on 587 or 465, with authentication.
When it shows up in your logs
Outbound port 25 from a non-mail-server host is a strong sign of a spam bot.