This kind of stuff is great - make suggestions, but allow it to go through even if you think it's wrong. For the last email validation I worked on, there were

This kind of stuff is great - make suggestions, but allow it to go through even if you think it's wrong. For the last email validation I worked on, there were only 2 absolute blockers - there must be an @ sign, and the domain must have MX records (emails that are technically valid remain useless if we can't send them anything). There were a number of other checks (being close to yahoo.com or gmail.com or other common email hosts, containing surprising characters, etc) that would trigger warnings, but still allow the check to pass if the user assured us it was correct.