Something like this:
MTA Checks the user database for existence of the gmuser@gmdomain before accepting the connection for the mail else refuse connection with error message. if the user exists, the mail is accepted or else refuse connection with error message. Obviously MTA has to have Mysql Auth compiled in, but what i was looking for was a step by step documentation on how to??? This becomes more relevant wrt false email addresses used by v*a*gra agents on net. It just unecessarily loads up the que.
The above instead of letting the mail in and then let GM handle the bounces wrt non existent user.