The default email client replaces MAPI32.DLL with a stub that redirects any calls to that DLL to itself. This isn't something unique to Thunderbird, that is the way Microsoft designed it. This can have side-effects. Outlooks calendar is implemented as a MAPI provider, so it needs to call the original MAPI32.DLL. This will break if you make Thunderbird the default email client. Novells Groupwise application only supports SimpleMAPI but it insists on being the default email client or it will not work. You can try to workaround this by copying the Groupwise MAPI32.DLL stub to the Groupwise directory. (When a application tries to find a DLL it normally searches in the current directory before it searches the windows system directory.)

Linux doesn't support MAPI. Wine supports SimpleMAPI but many of the APIs seems to be just stubs. [7]