Network ports · 587
587 — SMTP submission
Port 587 is the authenticated submission port for sending mail from a client.
Key facts
| Port | 587 |
|---|---|
| Transport | TCP |
| Service | SMTP submission |
| Direction | Client to mail server |
| Authentication | Required in practice |
| Encryption | STARTTLS |
| Range | Well-known (0–1023) |
Submission is not relay
Port 25 moves mail between servers; port 587 accepts mail from an authenticated user and hands it to the outbound queue. Separating the two lets a provider require credentials and rate limits for human-originated mail while still accepting inbound mail from the world.
RFC 6409 defines the submission protocol. Clients should authenticate and upgrade with STARTTLS before sending credentials or the message.
465 and provider quirks
Port 465 is implicit TLS submission: TLS starts immediately, without a STARTTLS command. Both 587 and 465 are used in practice; 587 is the standard submission port and 465 remains common because it avoids the plaintext window.
Provider documentation should be authoritative. Some block one port, some require an app password, and some rewrite the envelope sender. Test with your actual provider rather than assuming a universal configuration.
Frequently asked questions
Can I send mail on port 25 from a mail client?
It usually fails or is blocked. Use 587 or 465 for authenticated submission.
Does port 587 always use STARTTLS?
It should. A client must not send credentials before the upgrade succeeds.