Surely there are other less obtrusive ways to ensure the protection of the folder without "obscuring" the user's display. This is an example of an application "living outside of its envelope". Just because Microsoft has set a bad example in this regard is no reason to perpetuate it. :-)
It's fine to force a secondary window to be dominant within the application's primary window, but it should not force itselt to be "on top" of other applications' windows when Pmail is no longer in focus.
I think it would be good to reconsider this design decision.