Is it possible (and how) to set up two servers to handle mail for a domain (or two domains) in such a way that any of the servers can get down for a while (like few days) transparently, i.e. so that users will still be able to modify own mail over IMAP (like marking it as read, deleting, etc.)?
Getting the mail delivered in any way is easily done by setting MX records; but what mail storage can be used that way? In the best case it should be able to change data at both disconnected nodes and merge (with any way of conflict resolving) on synchronizations (couchdb comes to mind, although not completely applicable here).
I have exim4+courier-imap currently set up, but it won't be a critical problem to replace then with something.