Port 587
The TCP port for message submission (RFC 6409), where mail programs log in to their provider's server, normally upgrading the connection to TLS with STARTTLS.
What it means
A client connecting on port 587 starts in plain text, issues STARTTLS to encrypt the session, then authenticates with SMTP AUTH before sending. The server applies the provider's policies and relays the message onward. Google's SMTP relay service, for example, tells clients that use TLS to connect on port 587.
RFC 8314 now prefers implicit TLS on port 465 for submission but asks providers to support both during a transition, and RFC 5068 says access providers must not block port 587. A server on this port should refuse to relay for a client that has not logged in; Gmail answers 530 5.7.0 when authentication is required and missing.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 6409: Message Submission for Mailrfc-editor.org
- RFC 8314, section 3.3: Implicit TLS for SMTP Submissionrfc-editor.org
- RFC 5068: Email Submission Operations: Access and Accountability Requirementsrfc-editor.org
- Google Workspace Admin Help: Route outgoing SMTP relay messages through Googleknowledge.workspace.google.com
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