Triggering a Sendmail callback function

[braintree at worldnet.att.net (braintree at worldnet.att.net)]

I'm trying to trigger a callback function when a sending MTA receives a
"550 User not valid" response code after sending a "RCPT" command to the
receiving MTA.

1) Anyone knows whether the Milter filter APIs for Sendmail allow that
to be done?
2) If no API available, any one willing to give some pointers to a 
bootstrapped entrepreneur on where in the Sendmail code to put in a
callback function call?
 
Thanks very much
C.L.