Exch 5.5 SMTP Internet address problem

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

I changed the default reply address at my mailbox->Email Address and I add new Internet Address(domain abc) and set it as reply address.

If I sent alia@abc and alia@wxy I can receive both domain at my mailbox, but the problem is when to send it outside I still sent it as wxy although I have set the reply address as abc domain.

Any ideas, because I really have to fix this problem.

thanks
uji