When you have an e-mail data field and you have client-side validations turned on, OS built-in functions only check if you have an '@' written.

Well, that accepts strings like 'john@'.

By default, e-mail validation should check if there is a string, followed by an '@', followed by another string, a '.' and finally the last string, like every standard e-mail (something like this: 'john@something.com').