Welcome to the forums,
You really just need to tell us what web server software...
It could be as simple as a redirect page at
http://www.mydomain.net/webmail pointing to
http://mydomain.net:81
If you don't do it webserver side, put this in a simple html at mydomain.net/webmail:
<meta http-equiv="Refresh" content="0; url=http://mydomain.net:81">
If your already using apache for your other site these might stir your interest: (and they also tell you how to change the webmailport)
ZimbraApache - ZimbraWiki Zimbra with Apache using mod jk - ZimbraWiki Hosting other sites with Zimbra - ZimbraWiki Using Tomcat with Apache (mod jk) - ZimbraWiki