I don't think that would work.
What I have in mind: A backup solution where both servers are fully functional and fully synchronized between themselves, and none of them is actually just a small backup with less functionality.
E.g., 2 iRedMail servers, A and B. Both are identical as domains and user accounts, both have the same priority in the DNS, so both receive mail. Also, some kind of round-robin in the DNS, so users which have accounts on them connect randomly on A and B.
Now, if a mail gets delivered to A, it must be also visible (and physically present) very fast on B too. If a mail gets deleted on B, it should also be deleted on A and so on...
Can rsync do that? Let me raise a problem for example: For a user account, let call it John, the mailboxes on A and B are almost identical, with one difference: there is a mail message M1 only on A. Now, what does this mean? That message M1 just got delivered on server A, and B should get it too with rsync, or that the user connected to B and deleted it from there and A should delete it too?
How about quota? If mails get delivered on A and B, will the user quota get updated correctly?