Subnet and PortsPort 25: SMTP
Mail transfer between mail servers.
Registry entry
| Port | Protocols | Registered name | IANA description | Assignee | Reference | Note |
|---|---|---|---|---|---|---|
| 25 | tcp, udp | smtp | Simple Mail Transfer | IESG | [RFC-ietf-emailcore-rfc5321bis-43] |
What it is used for
Mail transfer between mail servers.
The risk
Open relays send spam; home networks are often blocked from using it.
What to do
Do not run a public mail server unless you will manage it. For sending from a mail program, use 587 or 465.
In the registry
Port 25 is a system port (0 to 1023), the range for standard services. The registry lists one service for this number. Its main entry is smtp, "Simple Mail Transfer", registered for tcp, udp.
How to find out whether it is open on your computer
On your own computer or server, ask the operating system which programs are listening for connections, and look for port 25 in the list. If a program you do not recognise listens on it, find out what it is before you close it. From outside, test only networks that you own or have permission to test: scanning other people's networks can be treated as an attack.
Is port 25 safe to leave open?
It depends on what is listening and who can reach it. Open only what you need, restrict by address where possible, keep the software updated, and use encryption and strong authentication. The advice above applies to this port.
Does the registered name prove what runs on port 25?
No. Programs can use any port. The registry tells you what the number was reserved for.
This is an independent guide. It is not connected with IANA, the IETF or any network equipment maker. Address and port data come from the IANA registries as fetched on 14 November 2026; RFC texts were read from rfc-editor.org. Check the originals for anything you rely on.