We have been getting alot of Returned Mail messages from email messages that we believe are being relayed through our server with Zimbra Collaboration Suite on it. These message are always spam and are either being rejected by a firewall (almost always barracuda) or because the intended recipient's email address did not exist. These spam emails are coming from all different IP Addresses so I can't simply block one or two IP Addresses. I upgraded our Zimbra Collaboration Suite from 7.1.3 to 7.1.4 and then to 7.2.0 in hopes that there would be a secuity patch that would fix that but that did not resolve the issue. I read through the following post: My Zimbra Sending Spam to The world. The instructions said to go into /opt/zimbra/conf/zmmta.cf and change the value of "smtpd_reject_recipents" to "yes". I went into this file and the exact same value was not there. However, I changed the value of "mtpd_reject_unlisted_recipient" from no to yes. I then restarted the Zimbra services. This did not resolve the problem either. I am not sure how to stop this relaying of spam mail through our Zimbra Collaboration Suite. Any suggestions would be appreciated.
1) Our Zimbra Collaboration Suite is the Open Source version and is version 7.2.0
2) The OS of the Server that our Zimbra Collaboration is on is CentOS release 5.8 (Final)