I am facing this problem from a long time with different versions of Zimbra. I had this problem with 5 so I upgraded to 6.0.2 but still the problem remains.
I have read that amavisd causes few problems. So disabled it and I am using an other anti-spam he problem doesn't look to die.
I have researched all the logs but I could see nothing unusual in both /var/log/zimbra.log and /opt/zimbra/log/mailbox.log
I have also seen in few forums that this is a bug in postfix. What should I do to get this resolved.