mx should check for unprocessed mail at startup

There's a chance that mx might fail or break and not process some mail. The flow might be that it fails to process a given mail, we notice because we have some monitor process or something, we fix it and restart the daemon. When it restarts, it should detect the unprocessed email, and.... well, process it.

(from redmine: created on 2013-05-20, closed on 2015-02-05, relates #6687 (closed))