I am on a 5.01 currently, however I did add alot of additional stuff, like postgrey, pyzor, razor, added alot of filters for emails accounts as well as distribution lists. Will I lose these settings?
As for the greylisting. In case of postgrey what happens is whenever an email comes in it is greylisted for 5 minutes and a 450 error is sent back to the sender forcing his mailserver to resend, once the second set of triplets match the mail is allowed through otherwise dumping the mail. This kills about 90% of my spam. I read in one of the wikis that in version 4.5 there was a GUI patch to see which actual emails/addresses were greylisted and in the end dumped. I found the following link:
Connecting with SQLGrey - Zimbra :: Wiki
This one uses SQLGrey which I would use if this was available in 5.x.