No option to validate for email in drupal webforms validation
Currently it seems there is no such functionality, as it still sits in issue queue as a feature request. You can either:


Use plugin from feature request, it's already there in issue queue.
Use regex validator, with one of the rules you choose. Simplest "usually OK" would be \b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b and you can find more at Regular-Expressions.info