how can I verify an email address is valid?

You can validate an email address by clicking on textbox element, then click on validation above and select email, check my Screenshot:

 

We also have a widget that you can use to spell check email addresses to prevent things like someone writing things like gmial.com:

http://widgets.jotform.com/app/email_correctness_check

 

I also added a new widget request to our widgets to do queue to check email addresses by sending an email to the user. If the user cannot confirm the email address he/she will not be able to submit it. We will inform you when it is added. 

 

thanks! that would be the perfect addition to getting valid emails addresses!
Dan
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