Hello Hartmut,
There's a good chance that this is being caused by a Mail Plugin or an issue with the Mail container. You can diagnose (and hopefully fix) the issue as follows:
These are plugins that have been installed to be system-wide, meaning that all user accounts on the Mac will have access to them through Mail app.
- Quit out of Mail app
- From the OS X Finder, hit Command+Shift+G and go to the following path:
/Library/Mail/Bundles/
- Locate the plugin to delete, typically named “PluginName.mailbundle” and drag it to the Trash
- Relaunch Mail app if finished
Next up, check the user Mail plugin directory. The path looks almost identical, but they are two separate locations on the Mac file system.
You will also want to check the user Mail Bundles directory, the process of uninstalling the user plugins is the same as above except the directory path is different:
- From OS X Finder, hit Command+Shift+G and target following path:
~/Library/Mail/Bundles/
- Delete the plugin as necessary (with a .mailbundle suffix) by dragging it to the Trash
- Relaunch Mail app
Rebuild the Container:
If Mail still isn't working, you can diagnose if this being caused by an issue with one of Mail's resource files by forcing Mail to rebuild them (this has nothing to do with rebuilding a mailbox). Quit Mail and move the following folder to the desktop (you can press shift+cmd+G from Finder and copy-paste if you need to):
~/Library/Containers/com.apple.mail
Restart your Mac. Open Mail again and let me know whether the issue re-occurs...
Max.