Port 587 — SMTP submission

Security guidance

This is the right port for sending mail from an application or client. Require authentication and TLS.

Test port 587 yourself

From your own machine, against any host (swap in your domain):

macOS / Linux — netcat

nc -vz example.com 587

Any OS — nmap

nmap -p 587 example.com

Windows — PowerShell

Test-NetConnection example.com -Port 587

open = a service answered on port 587; closed = the host replied but nothing is listening; filtered = a firewall silently dropped the probe. These test from your network — our port checker tests from the outside, the view that matters for security.

Other common ports