You do have a lot of options to add validation to classic Ui dialog
In classic all you need to do is add regex property with regular expression of email address
Refer complete documentation: https://docs.adobe.com/docs/en/cq/5-6/widgets-api/index.html
// regex : RegExp
Reference Post :- http://help-forums.adobe.com/content/adobeforums/en/experience-manager-forum/adobe-experience-manage...
I hope this would help you.
~kautuk