SUMMARY All running in the background, i.e. had just logged-in and nothing open Internet up and running In the background: korganiser sent an email reminder, which it did kmail created the email with the message but failed to send it email ends up in LocalFolders/outbox I have had this problem before, see additional information below STEPS TO REPRODUCE 1. 2. 3. OBSERVED RESULT email not sent EXPECTED RESULT SOFTWARE/OS VERSIONS Windows: macOS: Linux/KDE Plasma: (available in About System) KDE Plasma Version: 5.19.5 KDE Frameworks Version: 5.74.0 Qt Version: 5.14.2 ADDITIONAL INFORMATION I have had this problem before. Kmail seemed ok when used as a stand alone, the trouble began after installing Kontact. It was so bad I couldn't send any emails, they always ended up in the LocalFolders/outbox. Same error message was reported i.e. 'Failed to get D-Bus interface of mailfilteragent.' I wasn't able to isolate the problem to Kontact, Kmail or Akonadi. I uninstalled Kontact, no change. I removed all Kontact components and re-install only Kmail, problem remained. I purged all Kontact components, ran 'akonadictl vacuum' and reinstalled kmail only. Problem gone. Solely ran kmail for a week or so then one-by-one over a few days installed kaddressbook then korganizer. All ok until this incident.
I've run into this issue the other day for the first after many (mostly trouble-free) years with kmail + akonadi. What I had to do in the end to resolve it was starting up "akonadiconsole", switching to the "Browser" tab, opening the "outbox" folder on the left and then in the right pane deleting the "unsendable" message. It's been a few days since then and so far all seems to be good. So I'm just letting others known of a potential fix if needed. This was on KMail Version 5.23.0 (23.04.0) (copied from the About dialogue window) + Frameworks 5.105.0 + Plasma 5.27.4.