Once you have added an email address the system will check, if it is conforming to the conventional format of:  (something) @ (domain/website). The domain/website must actually exist.

Should you have added an email address, which is not conforming to the standard, you will see this message behind the field.

When you hover over the question mark at the end of the message you will get a general explanation, but not no specific details as to what needs to be changed/