Using "Mail Service" functions

I've noticed that some software - HPC Notes for example, and Pocket Outlook - has the option to send text out via email. Most of the time when you press the buton, or whatever, you just get Active Sync as the service, but since I installed Message-MasterSMS I get a choice.
This got me to thinking there must be a way to get the mail clients I *actually use* on this - Qmail and Npop - maybe via registry edit?
Does anyone have any ideas?
TIA
S

Go on - one of you clever dudes must know!