This one made me curious because updating the should be UI solved, and it is incredibly dangerous to mess with that. It is super easy to get random crashes when you invalidate the UI too much. It took me ages to get that "stable enough". But also technically an appointment request is a mail. And thanks to dan (afair), KMail can sign and encrypt invitations. And at least for signed invitations they are displayed as appointment so I looked into this a bit out of curiousity.

In T4836#132493, @aheinecke wrote:

Thank you for the detailed report.

We recently had a similar problem with S/MIME Mails. T4543 I think that we can apply the same fix we did for S/MIME also for OpenPGP. So I give this high priority as I think that this can be easily fixed and is a big problem in mixed environments.