Login to the control panel for your site. Once you have done this you will be presented a list of the users for your site. Click the envelope icon for the user taht you want to forward mail for. The page that loads will have three boxes. The top box is for your Forward Address. Put the address you want mail forwarded to. Note that if you have the user bob, with the aliases webmaster and sales, anything at bob@, webmaster@, and sales@ will be forwarded to the address you specify.