Port 25
The TCP port for SMTP between mail servers, used whenever one server delivers to another, for example from a sending platform to the recipient's MX host.
What it means
Every MX host listens on port 25, and every server delivering mail connects to it there; the MX system has no way to name another port. Connections on port 25 are normally unauthenticated: the receiver judges the sender by its IP, HELO name, SPF, DKIM and DMARC rather than by a login. TLS is added on the same connection with STARTTLS when both sides support it.
Because of abuse, many networks restrict outbound port 25. RFC 5068 describes access providers that block it, and Google Cloud, citing the risk of abuse, blocks connections from its virtual machines to port 25 outside the customer network unless a project is exempt. A tool that cannot reach port 25 should submit through an authenticated server on port 587 or 465, which then relays on port 25.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321: Simple Mail Transfer Protocolrfc-editor.org
- RFC 8314, section 7.3: Submissions Port Registrationrfc-editor.org
- RFC 5068: Email Submission Operations: Access and Accountability Requirementsrfc-editor.org
- Google Cloud: Sending email from an instance (port 25)cloud.google.com
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