Generic postfix bottleneck analysis:
Question for you: is your mail backing up because Zimbra LMTP is too busy or amavisd is not doing well (or clamav - check for CPU usage on your MTA box). What is the final destination of messages that are getting backlogged.
Also are there any spam bots in your network?
I ran yum update as mentioned in another thread and it seems to have fixed it. I'm not sure if it just needed the updated kernel or something else but I hope it can help some of you. This is thread I was referring to.