[Question] Spams sent from Zimbra MTA

[siavash]

Hi

I'm running a Zimbra server for a small group of users. Recently, I've noticed that daily mail reports show a large number of messages being deferred and bounced (few thousands daily).
In the mail queue, I can see that there are messages to random gmail users, sent by my local zimbra server, from random external accounts (in some cases emails are tagged as from:******@facebookmail.com, or from:******@linkedin.com).

It seems to me that some spammers have somehow got access to my zimbra mta server, and use it for spamming.

I tried to look into logs, but I couldn't understand how to track the issue.

I'll highly appreciate it if someone can give me a hint on how to resolve the problem.

Thanks