set reply mail add for SMTP internet add

I have Exch 5.5 and have 2 domain routed to it(1 domain – abc I recently added). I want selected user to have domain abc as reply address for smtp and the rest is domain wxy.
The problem is even though I have set domain abc as reply address for mymailbox, the email is still being sent as wxy domain.

Can anyone explain to me?

thanks
uji