[jeff@superman jeff]$ telnet 188.8.131.52 25
Connected to 184.108.40.206.
Escape character is '^]'.
220 mail.agarithil-nost.com ESMTP
250 mail.agarithil-nost.com Ok.
513 Relaying denied.
Now just make sure any "roaming" users have authenticated SMTP enabled in
their mail clients and you're set to go. (I'd also make sure they using
encryption, especially if they use LOGIN or PLAIN authentication.)
I suspect that he had a NAT firewall which forwarded all incoming port 25
connections to his server.
Naturally, everyone who would connect to the server would have, from the
server's perspective, the firewall's IP address, to which he granted