Hi mycwcgr,
You may first validate the mail address syntax is valid or not by JavaScript, and then use SMTP protocol to contact the domain mail server and ask if the user is valid. To do this, please refer to the sample code in the following link.
Effective Email Address Validation
http://www.codeproject.com/aspnet/Valid_Email_Addresses.asp
However, if you want to validate the password of that account is valid or not. I think Jos?s answer is the right way.
Sincerely,
Benson Yu
Microsoft Online Community Support
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