here are the steps:
1. create a record set that filters based on the post value, in your case it is probably the email.
2. then use the Dreamweaver Show Region Server behavior to show the div if the record set is not empty
3. then surround the DataAssist server behavior with the opposite logic, so you wanted if the record set is empty to run the datassist server behavior.
let me know if you have any additional questions.