I hve been looking for how to configure postfixs for over a week now, because I am converting over from a windows platform server.

I found these two sites that might help with the same issues I have had.

One site deals with postfix only.

http://www.yolinux.com/TUTORIALS/Postfix.html

But this one  deals with a complete installation of webmin and virtualmin

on a centos 5 box but should help with the configuration on a Turnkey Linux Box.

I have used a centos box with webmin using these instructions, and have now reinstalled Turnkey Linux because it is so much lighter. I just hope the 64bit version is out soon

http://jazzymarketing.com/main/0805/install-centos-5-server-webmin-how