The Dovecot documentation for the dbox storage format gives a couple dire-sounding warnings about corrupted or lost index files:
One of the main reasons for dbox's high performance is that it uses Dovecot's index files as the only storage for message flags and keywords, so the indexes don't have to be "synchronized". Dovecot trusts that they're always up-to-date (unless it sees that something is clearly broken). This also means that you must not lose the dbox index files, they can't be regenerated without data loss.
(...)
Note that with dbox the Index files actually contain significant data which is held nowhere else. Index files for both single-dbox and multi-dbox contain message flags and keywords. For multi-dbox, the index file also contains the map_uids which link (via the "map index") to the actual message data. This data cannot be automatically recreated, so it is important that Index files are treated with the same care as message data files.
So, I'm convinced that it's important be careful with them, back them up frequently, etc. But I would like to understand specifically what would go wrong if I fail to do so adequately, and unfortunately those blurbs are not specific enough for me to understand that.
I (think I) understand that the "message flags and keywords" are referring to IMAP stuff, but I don't really know that stuff in any detail. Is it just things like "After your server crashes and you restore your message data, but can't restore your indexes, clients connecting to the server will see all of their old messages as being unread, even though they read them previously"? That doesn't seem catastrophic to me - as long as the messages can still be accessed, I am basically fine with that. But if there are other problems that could arise in this situation, I'd like to know what they are.
Also, in the second quote, the additional specific warning for multi-dbox worries me. Am I understanding correctly that if I were to use multi-dbox, and preserved the message data files but not the indexes, I would be left with files that each may contain multiple messages without any clear indication of where one message ends and the next begins?
