Port 587 was set aside (RFC 6409) so that mail submission from people's devices is separate from server-to-server delivery on 25. That separation lets ISPs block 25 for spam control while you still send mail normally. Port 465, with TLS from the first byte, is officially recommended too since RFC 8314.
Facts
- Port
- 587
- Protocols
- TCP
- Service
- SMTP submission
What uses port 587
Email clients and apps submitting outgoing mail to the provider's SMTP server, which then relays it on port 25. Port 465 does the same with implicit TLS.
On a Mac
Apple Mail and other mail apps send outgoing mail over 587 (or 465). If sending fails while receiving works, check that 587 isn't blocked by the network.
sudo lsof -nP -iTCP:587Security
587 requires authentication and upgrades to TLS with STARTTLS. Make sure your client enforces TLS so the password isn't sent in clear.
Frequently asked questions
Should I use port 587 or 465?
Either, if your provider supports it. 465 uses TLS from the start; 587 starts plain and upgrades with STARTTLS. Both are secure when TLS is required.
Why can I receive but not send email?
Outgoing mail uses 587 or 465, which some networks block. Check the SMTP settings and try another network.
Is port 587 encrypted?
It starts unencrypted and switches to TLS with STARTTLS before the login. Clients should refuse to continue without it.