In Kmail2, Debian buster (stable) I try to delete a lot of unread messages from a folder. Let's say ~16000. But this is also reproducible with less, like 250 messages, or 100... Selecting the messages in the folder (Filter unread + CTRL+A + Shift+Del) - Filter unread messages - CTRL+A to select all - Shift+Del to delete permanently (because it is too long and buggy to move them to the trash) Then refresh the view : - click on Check Mail - Select the Maildir ressource containing the folder (it is a mixedmaildir_resource ) About half of the emails have been deleted only and the other hald is still remaining in the folder as unread. A new sequence of deletion will delete again only about half of the messages and so on... System: Debian Buster (stable) KMail 5.9.3 PIM/Akonadi 18.08.3