Change trash folder destination on account, then move messages in that account to trash. It goes to the previously set imap folder for trash for that account until kmail is restarted. Reproducible: Always
Which kmail version ?
Having/had the same issue in KDE 5.4.0 on Arch/Manjaro Linux. Might be related to: https://bugs.kde.org/show_bug.cgi?id=357945 (might contain additional hints)
Still happening reproduceably in KMail 5.4.2 (on archlinux). Changes to the trash folder location only take effect after a restart. EDIT: Can also more easily be fixed by restarting akonadi. Would still be sweet if it was working without having to do this. ;)
*** Bug 375136 has been marked as a duplicate of this bug. ***