I think Zimbra is generating spam and causes our ip to be black listed almost every day.

We are using Trend Miro Interscan for viruses and it forwards all mail to our Zimbra server, and all out going mail from Zimbra goes to the interscan server. That way we are scanning for viruses in and out.

The problem is that we are getting spam that is coming in and Zimbra replyes to the messages because the email don't exist. Then we get a reply back from what ever domain saying the email that we tried to send to doesn't exist.
At least that is what I think is happening as far as I can tell.

Does that make sense? How can I change the settings in Zimbra to not reply to emails that are coming to invalid addresses.

We get alot of emails for example to say bob@ourdomain.com bob is not a valid email address. Zimbra replys that its not valid, and then we get emails back saying the other doesn't exist.

I have been looking at the settings in MTA and am wondering what would be the effect of checking

Client's IP address (reject_unknown_client)
Hostname in greeting (reject_unknow_client)
Sender's domain (reject_unknown_sender_somain)

All of which are unchecked.

Anti-Spam is enabled with 75/33

Thanks for any help