Postfix is installed by default on the Drupal 5/6 appliances and should handle sending Drupal messages (e.g., password recovery) without further configuration.

To configure postfix to relay mail to another mail server (e.g., Exchange), set the value for "relayhost" in /etc/postfix/main.cf like this:

sed -i '/relayhost/ s/=.*/= [my.exchangeserver.net]/' /etc/postfix/main.cf

Alternative you can just edit the configuration file with a text editor (e.g., vim).

Unfortunately, there doesn't seem to be an easy way of doing this from the webmin interface.