Had a couple of recent cases of not being able to open the Control Panel Mail applet (either from Control Panel directly, or via command line etc). The general consensus seems to be that a recent MS update (quelle surprise!) has broken it.
In addition to the suggested “fixes”, if you need Outlook to prompt for a profile each time it is run, then you can force it to via the registry:
Value: 1 (= Prompt for a profile to be used; 0 = Always use this profile)
Note that I had to add the Client and Options sub-keys, before finally creating the PickLogonProfile string value.