When it comes to email validation, I want either :

• A very permissive regexp : I will check validity by sending a mail. I will get tons of false positive.

   ^\S+@\S+$

• A more restrictive regexp : I will get few false positives, and few false negatives. It's accurate with most (like 99,9%) of the mails in use.

 [A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}

Whenever possible, send a validation mail

That's how I would proceed with a person (customer) over the phone, that's how I expect my app to behave :

• I sent a 'validate adress' mail. Please confirm it's your mail by clicking on the link you will find in the mail.
  .If you did not recieve the mail ... (user's role to do something about it).

On validation :

• Thank you + message & useful information related to message.