Zimbra as NDR Spam source
I've noticed one of the domains that I am hosting is starting to get random emails sent to it. The emails are sent to something like "firstname.lastname@example.org", they claim to come from an address that looks innocent enough, but the server sending the email is completely unrelated to the source address in the "from" field.
This is causing zimbra to bounce the email with "sender non-delivery notification".
To me, this looks like I may be the source of NDR Spam... someone sends a bogus email to my domain as some legit email victim, and when I reject the email, the victim gets the bounce message for an email they never sent.
How can I stop this? Is there a per-domain way to turn off "user doesn't exist" bounces and just disregard them?
Some of the domains I host have "catch-all" addresses and others do not. I don't have the luxury of turning off or turning on catch all accounts for all my domains, they must be configured individually.
I despise spam, especially NDR spam so I really don't want to contribute to the problem.
Any help would be appreciated.
Thanks to you all in advance,