in reply to email programs ????
Well, not that I can think of. It all depends on what you configuration is. This is such an incredibly vague question!! Suppose you had sendmail doing smtp and were using qpopper on a completely different machine to do POP ?
Methinks that you should do some more investigating into this....... >:(
Once you can answer questions like "What is my smtp server?" then writing a script to add a user is very quick work.