- Port
- 25
- Protocol used by the service
- TCP
- IANA registrations
- TCP/UDP
- Encryption
- Varies by configuration
- IANA description
- Simple Mail Transfer
- IANA service name
- smtp
Note: IANA has formally registered SMTP on TCP/UDP, but in real-world practice this service operates over TCP only — a formal registration on a transport doesn't guarantee the protocol is actually implemented or used over it. This page's Protocol field reflects genuine usage, not just the registry entry.
What port 25 is used for
Server-to-server email transfer between mail servers. Not typically used by end-user email clients for sending mail — that is usually port 587 (submission) or 465 (SMTPS) instead.
Is port 25 encrypted?
Not encrypted by default, but can be upgraded in-session via STARTTLS. Whether a given connection ends up encrypted depends on both mail servers supporting and enabling it.
Firewall & exposure considerations
A mail server needs 25 open to receive mail from other servers, but a misconfigured or compromised server relaying on 25 can turn into an open relay for spam — outbound port 25 is blocked by default on many ISPs and cloud providers for exactly this reason.
Related ports
References
Look up any other port
Search the full IANA registry — all 6,100+ ports and 12,500+ protocol-specific records — on the Port Lookup tool.
Last reviewed: . Data source: IANA Service Name and Transport Protocol Port Number Registry. Security and usage notes are this site's own editorial content, not official IANA data.