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TCP
Unknown
25 SMTP
SMTP routes outbound mail between applications, relays, and mail transfer agents.
Security Context
Port 25 is context-dependent. Its risk depends on the daemon behind it, whether the traffic is encrypted, and how tightly the listener is scoped to trusted networks.
- !Misconfigured relays can become open-relay spam sources.
Nmap Check
Use a version probe first so you confirm whether the expected service is really what is listening on this port.
nmap -sV -p 25 <host>Firewall Commands
UFW
sudo ufw allow 25/tcpiptables
sudo iptables -A INPUT -p tcp --dport 25 -j ACCEPT