Only if I shift-del delete mail from the inbox is the mail really gone; if i press del, it's copied to trash. It's no longer shown in the inbox list, but if I go to another folder, then back, the mails are back, too. Reproducible: Always
did you enable "automatically compress folders" in the settings dialog of your akonadi imap plugin?
I'm not using imap.
This bug has only been reported for versions older than KDEPIM 4.14 (at most akonadi-1.3). Can anyone tell if this bug still present? If noone confirms this bug for a recent version of akonadi (part of KDE Applications 15.08 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.