Hello kmail team,
after using kmail till ~4.9 I switched to Thunderbird due to some annoying bugs. I wanted to give the new 4.12.3 version a chance, so I started up kmail and connected my IMAP account, because I had deleted the account from Akonadi when I switched to Thunderbird. Although I have deleted it, some of my settings were still present like filters, expiry dates etc. which shouldn't be a bad thing in the first place.

The problem is, that although none of the IMAP folder names have changed, kmail mixed up the folders, e.g. a filter which should move new mails like Logfiles to Administration/Logfiles it moved them to ProjectXY/ImportantStuff . The real problem is, that the same thing happened for the expiry dates. The folder Administration/Logfiles had an expiry date of 30 days, the folder ProjectXY/ImportantStuff had none. As the two folders were internally swapped all old mails in ProjectXY/ImportantStuff got deleted.
This did not just happen to two folders but kmail rampaged through my whole IMAP account. When I noticed it was already too late.

I haven't tried to reproduce this and will not try, but I suspect kmail is storing the info for the folders not by the folder name but maybe by some Akonadi ID (?),  and the ordering of those IDs was changed when I readded the account.
I think this is a highly critical and dangerous bug. While not appearing for everyday use it can cause tremendous data loss.

Reproducible: Didn't try

Actual Results:  
Filters move files to wrong directories, expiry dates are applied on the wrong directories causing data loss.

Expected Results:  
Either filters / expiry dates should be removed when the IMAP account is removed or a method should be implemented to ensure a correct association of the filters to the IMAP directories, once the account is readded (e.g. a simple identification by name would do the trick).