Have a separate helper EXE that takes command-line params (or pipe to its StandardInput) that does what is required and call that from your main app. This keeps the MAPI stuff outside of your main app's process space. OK, you're still mixing MAPI and .NET but in a very short-lived process. The assumption is that MAPI and the CLR start causing issues with longer-running processes.
We use Dmitry Streblechenko's superb Redemption Data Objects library which allows us to write such "shim" code in JScript and invoke that, which keeps the CLR and MAPI worlds in separate processes, but in a supported fashion.
@Chris Fournier re. writing an unmanaged DLL. This won't work because the issue is mixing MAPI and managed code in the same process.
