Experiment 1 1. when a mail arrives then immediately display its contents and put it in trash 2. in any folder try to display any mail contents Result : you get a message saying please be patient kmail is retrieving the mail For every restart of kontact then you get a crash. You need to restart akonadi with "akonadictl restart" to avoid a crash Experiment 2 1. when a mail arrives wait for the filtering process is finished 2. display the mail contents Result : No problem Conclusion : I suggest kmail does not display a mail before finishing the filtering process. SOFTWARE/OS VERSIONS Windows: macOS: Linux/KDE Plasma: (available in About System) KDE Plasma Version: 5.20.5 KDE Frameworks Version: 5.78.0 Qt Version: 5.15.2 kde apps 20.12.2
another problem: if a mail is displayed before filtering then it is a dangerous mail because it is not virus checked and spam checked.