I no longer consider myself a newbie webmin/VM user. Here is the situation. I installed WM/VM pro on a clean CENTOS 5.2 install. Postfix was installed as the default MTA by the install script. I want to use sendmail because that what I've been using on several other servers that I maintain. I configure all the other programs, http, dns, ftp..etc. Everything works fine. Then comes mail which is very important! I try to turn off postfix and turn on sendmail from within WM/VM pro. Doesn't work. I use an external switch utility to change MTA to sendmail, then I am able to turn off postfix and startup sendmail. Mail still dosen't work. Mail logs suggest that postfix is still embedded in several programs needed for mail to function. I remove Postfix module from within WM/VM pro and reinstall sendmail module. Now a lot of stuff dosen't work.
How can I run the WM/VM pro install script and have sendmail be my MTA!!

The steps to switching aren't all that troublesome (and both can be installed simultaneously on CentOS).

OK, so you start with a fresh install of everything.

Disable Postfix:

service postfix stop
chkconfig --level 345 postfix off

Enable Sendmail:

service sendmail start
chkconfig --level 345 sendmail on

I suspect you also have to tell Sendmail you'll be doing virtual hosting...but I don't know how that's done.

Use alternatives to setup the symlinks for various command line tools:

alternatives --config mta

Set it to sendmail.

Tell Virtualmin to use sendmail in the Module Configuration.

Re-check configuration.

That's it. You're done. Nothing else should be needed...and there aren't going to be any situations of "postfix still embedded in several programs". Virtualmin dynamically figures out most of the stuff needed for mail delivery...the only thing that isn't dynamic and changeable at will is the virtual maps table. ;-)

After doing this, feel free to report back with log entries if things remain mysterious.

Don't panic, or get all willy-nilly in your actions. It's designed to be easy to switch (until you have users...after which it is time-consuming and non-trivial).