It is the location of the mail.php file that is installed with PEAR. In most cases it can be blank because when Pear is installed properly it can almost always be blank... 99% of the time blank but who knows with your host ;)
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.