Nessus finds my mailserver to be an open relay:
Here is a trace of the traffic that demonstrates the issue :
S : 220 my.mailserver.at ESMTP
C : HELO example.edu
S : 250 my.mailserver.at Ok.
C : MAIL FROM: <test_1@example.edu>
S : 250 Ok.
C : RCPT TO: <test_2@example.edu>
S : 250 Ok.
C : DATA
S : 354 Ok.
but I don't find the setting where I could close it. The output can be reproduced with telnet, so I presume that's a valid vulerability.
ESMTPAUTH is enabled, so I figured, according to the documentation, relaying should only be permitted for authenticated users (so mails can be sent).
On the other hand, https://mxtoolbox.com checks ›OK – Not an open relay‹, http://www.aupads.org/ tells me (according to answers like the ones shown above), the server has accepted a mail for relaying but may or may not actually do it.
I skimmed through courierd esmtpd esmtpd-ssl and imapd-ssl config files, but would not find what setting(s) I have to adjust.
What am I missing? What can I provide to help track that down?
Add: That's the entire report for that vulnerability: Nessus: MTA Open Mail Relaying Allowed