Accessing Webmail without using port No. extension

Hi All,
I've web server and mail server running on same Windows 2k3 + IIS 6
box.

Mail server is Code Crafters Ability Mail Server.

I can access mail server by using http://www.mydomain.com:1234

But I want to access mail server by just typing URL of domain without
port extension
E.g. http://webmail.mydomain.com

I tried 2 solutions
1) Host header solution: I configured webmail.mydomain.com hos header
and pointed port to 1234
- This obviously fails as I'm trying to use port 1234 which is being
used by mail server

2) I configured web