When you install Virtualmin GPL by install.sh you can found this problem:

INFO - 2008-08-22 07:50:42 - Removing Debian standard Webmin package, if they exist (because they're broken)...
INFO - 2008-08-22 07:50:42 - Removing Debian apache packages...
DEBUG - 2008-08-22 07:50:43 - Reading
INFO - 2008-08-22 07:50:43 - Installing dependencies using command: /usr/bin/apt-get --config-file apt.conf.noninteractive -y --force-yes install postfix postfix-tls postfix-pcre webmin usermin ruby libapache2-mod-ruby libxml-simple-perl libcrypt-ssleay-perl unzip zip
Reading package lists...
Building dependency tree...
Package postfix-tls is a virtual package provided by:
postfix 2.3.8-2+etch1
You should explicitly select one to install.
FATAL - 2008-08-22 07:50:44 - Fatal Error Occurred: Something went wrong during installation: 0
FATAL - 2008-08-22 07:50:44 - Cannot continue installation.

This is because postfix-tls is now a virtual package, TLS support in postfix is now built in main "postfix" debian packages.

Edit install.sh and delete postfix-tls package from it.

Ciao