Port 25 (TCP) — SMTP

Mail transfer between servers.

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Make sure it is not an open relay. Many cloud providers block outbound 25 by default.

Check if port 25 is open

On the server, see what is listening:

sudo ss -tulpn | grep ':25 '

From another machine, test reachability:

nc -zv your-server.example.com 25

With UFW (Ubuntu/Debian):

sudo ufw deny 25

With firewalld (RHEL/AlmaLinux/Rocky):

sudo firewall-cmd --permanent --remove-port=25/tcp && sudo firewall-cmd --reload