There may be a more "webmin'y" way of accomplishing this, but I have had luck doing this using mod_rewrite.
What you can do is add code to your httpd.conf file, which looks for a particular string in the url the browser is going to, and if it exists, redirect to another url.
So in this case, take the following code:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^webmail..*$ [NC]
RewriteRule ^(.*)
http://your_domain.com:2000The above will look for url's beginning with "webmail.", and upon seeing it, will redirect to "your_domain.com:2000".
You can of course explore how to use $1 rather than hardcoding "your_domain.com" if you want to preserve the domain name the user typed in.
More docs on mod_rewrite here:
http://httpd.apache.org/docs/2.2/mod/mod_rewrite.htmlGood luck!
-Eric