have you monitored the /opt/zimbra/log/mailbox.log for all activities of such users?
if there's one email going to this user, it should have log something like :
Code:
2009-09-11 00:05:25,289 INFO [LmtpServer-346792] [name=xxx@xxxx;mid=xxx] mailop - Adding Message: XXXXXXXXXXXXXXX
mail will reach user's mailbox when you see this message.
and don't forget to check the queues, maybe someone are sending lots of email at that time and it surely will delay the email behind.