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

A JavaScript RegExp object to be tested against the field value during validation (defaults to null). If the test fails, the field will be marked invalid using regexText.

Reference Post :- http://help-forums.adobe.com/content/adobeforums/en/experience-manager-forum/adobe-experience-manager.topic.html/forum__wfdy-i_have_gonethroughh.html

I hope this would help you.

~kautuk