Double check with your host the pear Mail package has been installed on the server.
this error suggests that pear is installed, but either the Mail addon package is not installed, or your path to pear is not correct.
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.