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.
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