Hello!
Due to a recent change in IMAP server structure some directories where moved on the server. Sadly my akonadi install wasn’t able to upload my sent messages to the server for some time, so the mails were cached on the disk, but this change in directory structure led akonadi to delete all cached sent mails in file_db_data (even if it wasn’t even in a touched directory!), making me loose all sent mail since mid february (why is it even doing this?) . Now I’m trying to extract as many mails as possible from the akonadi database, but while I do know what to do with simple text mails, I’ve no idea what to do with the multipart mails. These feature a second database entry containing binary data. Does anyone of you have an idea how these mails could be extracted from the database?

Mails aren't stored in the database, unfortunately. If they're larger than 4K, they're stored on the filesystem. For your error (and deleted mail) I would suggest to file a bug on bugs.kde.org.

I can't see an easy way to recover the deleted mails, unfortunately.