Hello,
     I think the problem you are facing can be very well solved through 
Remote Scripting tech. You can execute a asp page on server and get the 
status back to the present page without refreshing the page. You can check 
for the duplicate email, or validate the email or validate the domain, or 
validate the email itself by sending a token to SMTP server of the 
concerned domain name and get the results back to the present page in 
Javascript and can display it accordingly to the user.

 For example: if the user name is john@o..., you can pass that 
email to a asp page which will run in a backend process where you can 
execute any ASP scripts and get the value back in the current page through 
javascript.

  You can get the whole picture about this Remote Scripting at the 
following URL


http://msdn.microsoft.com/library/default.asp?url=/library/en-
us/rmscpt/html/rmscpt.asp

The MSDN articles are self-Explanatory. In case of any clarification feel 
free to cntact me
Regards

Shashank
Analyst, DTDC Courier and Cargo Ltd.