Reverse proxy with URI rewrite

Hi, sorry for posting this kind of question again, as it looks like there would be enough ogf them already. :(
Well I'm stupid enough not to catch the point. I just to make this work, but it doesn't anymore.

I want requests for
http://webmail.domain.com
I want the Reverse Proxy service to fetch it from this address
http://webmail.domain.com/sub/folder

Thats all!