Port 25 — SMTP

Security guidance

Needed on a mail server, pointless elsewhere. Many home and cloud providers block outbound 25 to limit spam.

Test port 25 yourself

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

macOS / Linux — netcat

nc -vz example.com 25

Any OS — nmap

nmap -p 25 example.com

Windows — PowerShell

Test-NetConnection example.com -Port 25

open = a service answered on port 25; 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