You can usually leave it blank. "localhost" is also a common value to use... it could also be an IP address or mail.domain.com address depending on the implementation.
90% of the time blank will work.
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.