For our company, using separate accounts with zimbra is not useful because they share a lot (and the sharing of folders is too limited, since conversations and tags don't work over them). Therefore, I've set up one account with different folders acting as virtual inboxes.
To be able to have multiple instances of one message spread across these virtual inboxes, I had to disable Zimbra's duplication detection:
zmprov mcf zimbraMessageIdDedupeCacheSize 0
However, this yields some unpredictable behavior, namely that the conversation is only tracked in one of the virtual inboxes. The incoming duplicate mail can be found in both folders, but the conversation can only be seen in one of the virtual inboxes (and in the sent folder).
I checked the source and the message-id's en in-reply-to headers are all OK.
Does anyone have a solution for this?
Edit: because this seems like a bug, I filed a bug report.