587 SMTP-submission TCP
Reference for TCP port 587, commonly associated with SMTP-submission.
What it's for
Port 587 is the standard port for SMTP mail submission — the correct port for a mail client or application to use when sending outgoing email through its configured mail server. This is distinct from port 25, which is meant for server-to-server relay across the internet; port 587 exists specifically to require authentication for the "a person or app is sending mail" case, which port 25 was never designed to enforce.
Protocol basics
Port 587 uses TCP and typically starts as a plaintext connection that's then upgraded to TLS via the STARTTLS command before any credentials or message content are transmitted. Authentication (username/password, or an API key with services like SendGrid/SES) is required before the server will accept a message for delivery, which is the key security distinction from plain, unauthenticated port 25 relay. This is the port you'll configure in almost any transactional email setup — application servers, CI/CD notification systems, and standard email clients alike.