I've also experience with this problem on one of my client Zimbra server. Zimbra is not an open relay by default, and it's true but it is not enough to protect Zimbra from sending massive fake email from client on the trusted network. I think the problem are on the workstation, that has infected by trojan or virus that sending massive mails to outside by relaying to Zimbra. Most of the email recipient are fake and Zimbra will deferred the mails but the proses taken too much resources.
I suggest the following suggestion :
1. Tracking the deferred mail source and find the IP who send the fake mail.
ex :
Check the queue ID and open the message source with postcat :
Code:
/opt/zimbra/postfix/sbin/postcat
/opt/zimbra/postfix/spool/deferred/groupID/queueID
Ex :
Code:
/opt/zimbra/postfix/sbin/postcat
/opt/zimbra/postfix/spool/deferred/D/D125A828AF0
If you find the original IP who send the massive email, scan it with your updated anti virus/trojan.
Other solution are creating the SpamAssasin rule as you mentioned on the thread title, but it was not my knowledge :-(