Bounce Mail Woes

Hi,
In our setup we are using one server (let's call it mail.abc.com)
as the server having user mailboxes and another server (let's call it
gateway.abc.com) as server directly connected to internet.
gateway.abc.com is also hosting DNS for domain abc.com and has entry
gateway.abc.com as MX.
mail.abc.com relays all mails to gateway.abc.com through
SMART_HOST. A virtusertable file is maintained on gateway.abc.com
which maps all the mails to users@abc.com to users@mail.abc.com.
Now at present every thing else apart from bounce is working fine.
I'm able to send mails to local users on mail.abc.com through
gateway.abc.com and also to users external to abc.com domain. However
if I've written a wrong recipient address then I don't receive bounce
mail for this.
I checked the log file on gateway.abc.com and found that the server
queued the mail. It did try to bounce back the message to me (let's
assume my user id is xyz@abc.com) to xyz@abc.com but the connection
was refused by abc.com for no particular reason.
I can get bounce mail if i send a mail to an invalid user in
abc.com domain, however for invalid users on other domains or for
invalid domains I am not able to receive any bounce mails. All the
bounce mails are landing in queue and after 5 days they are getting
dropped to postmaster@abc.com (postmaster@abc.com) is a local account
on gateway.abc.com.
It would be very kind of you if you can help me out with this
problem. I'd be more than willing to share my configuration files etc.
provided some feedback is provided to my problem.

Regards,
Mayank