One of my clients is using a cPanel-based Virtual Dedicated Server that appears to allow unauthenticated SMTP:
bash$ echo EHLO | nc mail.clientscompany.com 25 ... 250-SIZE 52428800 250-PIPELINING 250-AUTH PLAIN LOGIN 250-STARTTLS 250 HELP
It therefore appears that anyone (esp. spammers) can use his mail server to send whatever - I just connected from my DSL connection at home, and...
bash$ nc mail.clientscompany.com 25 HELO clientscompany.com MAIL FROM: firstname.lastname@example.org RCPT TO: email@example.com DATA From: <firstname.lastname@example.org> To: <email@example.com> Date: ... Subject: ... Blah . QUIT
I just tested this, and sure enough, it sent a mail from "firstname.lastname@example.org".
Since I am not familiar with cPanel and WHM, can someone provide pointers to configure his mail server to (a) only accept TLS connections and (b) only authenticated ones (i.e. with user/password, not just plain connections).
Thanks for any help.