what is your version of Dreamweaver?
Do you get this error when going opening the Create Email Message server behavior for the first time? Or when editing an existing Universal Email instance? or when clicking OK inthe Create Email Message server behavior? Please provide details on the workflow leading up to the error message.
In your Dreamweaver Site definition, do you have a testing server defined? A testing server should be defined.
see the "Defining a Dynamic Site in Dreamweaver" tutorial on the learning resources page.