Port 25: SMTP
Mail transfer between mail servers.
- Port
- 25
- Protocol
- TCP
- Service
- SMTP
- Exposure
- Expose only with access controls
What is port 25 used for?
Receiving and relaying email between domains.
Is it safe to expose port 25 to the internet?
Open relays get abused for spam. Expose it only on your mail server and require authentication for relaying. Many ISPs block outbound port 25.
If port 25 seems closed or unreachable
- Check that the SMTP service is running and listening on port 25, not only on 127.0.0.1.
- Check the firewall on the host itself (Windows Defender Firewall, iptables, nftables, ufw) for a rule that allows TCP 25.
- If the host is behind a router, confirm the forwarding rule points to the correct internal address and that your internet provider does not use carrier-grade NAT.
- Test from outside your own network. A port can be open locally and blocked at the edge, or the other way round.
Using port 25 safely
Port 25 can be exposed when the host exists to provide this service, but add controls: restrict source addresses where you can, require strong authentication and encryption, and monitor the logs for repeated failures.
Check whether port 25 is open on your host
The check connects from our server, so it shows what the internet can reach.
Open the port checker