Since a server reboot yesterday, I'm getting a very very annoying by on my Zimbra 8.0.1 (network edition).

Whenever zimbra receives an e-mail over LMTP, the following happens:
2012-11-30 15:08:44,253 INFO [LmtpServer-3] [ip=;] lmtp - Delivering message: size=4434 bytes, nrcpts=1,, msgid=<>
2012-11-30 15:08:44,257 INFO [LmtpServer-3] [;mid=3;ip=;] mailop - Adding Message: id=1453646, Message-ID=<>, parentId=-1, folderId=4, folderName=Junk.
2012-11-30 15:08:44,306 WARN [LmtpServer-3] [;mid=3;ip=;] lmtp - try again for message,to=stgraber@stgra
at com.zimbra.cs.lmtpserver.ZimbraLmtpBackend.addToDe dupeCache(
at com.zimbra.cs.lmtpserver.ZimbraLmtpBackend.deliver MessageToLocalMailboxes( )
at com.zimbra.cs.lmtpserver.ZimbraLmtpBackend.deliver (
at com.zimbra.cs.lmtpserver.LmtpHandler.processMessag eData(
at com.zimbra.cs.lmtpserver.TcpLmtpHandler.continueDA TA(
at com.zimbra.cs.lmtpserver.LmtpHandler.doDATA(LmtpHa
at com.zimbra.cs.lmtpserver.LmtpHandler.processComman d(
at com.zimbra.cs.lmtpserver.TcpLmtpHandler.processCom mand(
at com.zimbra.cs.server.ProtocolHandler.processConnec tion(
at java.util.concurrent.ThreadPoolExecutor.runWorker(
at java.util.concurrent.ThreadPoolExecutor$ (
2012-11-30 15:08:44,307 INFO [LmtpServer-3] [;mid=3;ip=;] lmtp - S: 451 4.0.0 Temporary message delivery failure try again (DATA)
2012-11-30 15:08:44,307 INFO [LmtpServer-3] [] lmtp - Handler exiting normally

This is happening for every single e-mail the server receives. The e-mail is correctly added to the pool and I can see it in my mail client, however as the server returns a 451 to my inbound SMTP server, the e-mail delivery is attempted over and over again, leading to a ton of duplicate e-mails.

So far I've tried:
- Upgrade to 8.0.1 (I was on 8.0.0)
- Remove all the indexes and rebuild them for all accounts
- Turn off the deduplication feature in the COS and for all accounts
- Set the deduplication cache size to 0 (which should turn it off completely)
- Checked the DB integrity

The server still has plenty of free disk space, plenty of free inodes and 25GB of free memory.

Any idea?