When I was in charge to implement the email validation at my previous employer, the only thing I did was checking for "." and "@" in the correct order. The sal

When I was in charge to implement the email validation at my previous employer, the only thing I did was checking for "." and "@" in the correct order. The sales-focused part of the company complained that they would still receive invalid email-addresses for their newsletter. Turns out what they really wanted was an email address where they could send stuff to. So we implemented a MX-check for the host part of the email address. However, that check was mostly a warning, in the sense of "Hey, are you sure the email you entered is correct?".