In these days I noticed I lost a lot of e-mails. Apparently mail are sent correctly, but they haven't been delivered in users' mailboxes.
If I check mailbox.log I find this:

2009-04-24 12:24:13,413 INFO [LmtpServer-372] [ip=x.x.x.x;] lmtp - Delivering message: size=2971 bytes, nrcpts=1, sender=user1@mydomain.com, msgid=<!&!AAAAAAAAAAAYAAAAAAAAAJS9LDrQ/uNBqj6uTL1KlK/CgAAAEAAAAEPV9VWF5pdKjxsLLYhdYSUBAAAAAA==@mydomain .com>
2009-04-24 12:24:13,413 INFO [LmtpServer-372] [name=user2@mydomain.com;mid=20;] lmtp - Not delivering message with duplicate Message-ID <!&!AAAAAAAAAAAYAAAAAAAAAJS9LDrQ/uNBqj6uTL1KlK/CgAAAEAAAAEPV9VWF5pdKjxsLLYhdYSUBAAAAAA==@mydomain .com>

I noticed this problem in two different zimbra server.

The bad thing is that user are not aware of undeliverable mail, If i didn't check log after users complaint I wouldnt' be aware of that.