QMail setup

I’m just successfully installed RT3.0.5 with RH9 + Postgres + FastCGI +
Apache2. Currently it works fine except for some minor problems (httpd
log error).

I installed it in a one of local linux machine with Sendmail (private IP
setup) because I will use this for our intranet then I have another linux
machine where our Qmail mail server is running (with public IP).

Generally my question is what should I do for this kind of setup?

Right now what happen the requestor can received email from RT but RT can
not pick up email from requestor (comments, etc.). Why the domain name is
still @localhost.localdomain from the email address coming from RT?