So maybe not DirectX.
I noticed that if I click “New Event > Online Meeting” I get the prompt saying “No online meeting providers set up. Set one up?” or whatever it is exactly. Clicking yes opens the settings but in a different section. Form there I can navigate through every settings section, except general.
I also noticed that launching using proton instead of steam, I’m able to open the settings. On the general section, there is the default application button. Clicking this in an instance launched with proton causes a crash.
So possibly something to do with default application/protocol handlers.
Some more notes:
Installing the Calibri font into the Wine prefix and setting eM Client to use that instead of Tahoe fixes the jaggy font issue.
PGP import, encrypt, sign and decrypt all work under Wine.
Using the Nextcloud cloud attachment functions works too.
When launching with proton, which allowed me to access the general settings tab, I was able to set the application to close to the system tray, which indeed closed it to the KDE Plasma system tray. Re-opening it from there worked.
Unfortunately launching with proton results in the application having no network connectivity, something I’m not willing to bash my head against right now.