# WHATPORT(25)

## NAME

port 25 — SMTP — SMTP between mail servers — how mail is delivered host to host, not how your mail client submits it.

Protocol: TCP  
IANA registry

## DESCRIPTION

A sending MTA looks up the recipient domain's MX record and connects to port 25 there, then runs EHLO / MAIL FROM / RCPT TO / DATA. Modern sessions upgrade to TLS opportunistically with STARTTLS (RFC 3207). Clients should not use 25 for sending their own mail: authenticated submission is port 587 (or 465). Most residential ISPs and cloud providers block outbound 25 by default to limit spam, so a failed connection is often the network, not the server.

## EXPOSURE

**Expose with care.** An internet-facing MX on 25 is normal and required, but it must never relay for unauthenticated senders outside its own domains.

## CHECK

```sh
printf 'EHLO test.example\r\nQUIT\r\n' | nc -v aspmx.l.google.com 25
```

## SEE ALSO

- spec — [RFC 5321 — Simple Mail Transfer Protocol](https://www.rfc-editor.org/rfc/rfc5321.html) (rfc-editor.org)
- spec — [RFC 3207 — SMTP Service Extension for Secure SMTP over TLS](https://www.rfc-editor.org/rfc/rfc3207.html) (rfc-editor.org)
- docs — [Postfix documentation](https://www.postfix.org/documentation.html) (postfix.org)
- iana — [IANA Service Name and Transport Protocol Port Number Registry](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml) (iana.org)

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HTML: <https://whatport.net/25/> · JSON: <https://whatport.net/25.json> · full dataset: <https://whatport.net/ports.json>