Port 25: SMTP
Port 25 is SMTP, used to relay mail between servers. Many providers block it on residential lines to fight spam; mail clients submit outgoing mail on port 587 or 465 instead.
TCPEmailUnencrypted by defaultIANA assigned
| Port number | 25 |
|---|---|
| Service | Simple Mail Transfer Protocol |
| Protocol | TCP |
| Category | |
| Default encryption | No |
Security note. Port 25 is not encrypted by default. Restrict it to trusted networks, or use an encrypted alternative where one exists, to avoid exposing credentials and data in transit.
Frequently asked questions
What is port 25 used for?
Port 25 is SMTP, used to relay mail between servers. Many providers block it on residential lines to fight spam; mail clients submit outgoing mail on port 587 or 465 instead.
Is port 25 TCP or UDP?
Port 25 (SMTP) uses TCP.
Is port 25 secure?
Port 25 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 25 on my firewall?
Only if you specifically need SMTP. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.