Why go to the point of trying to send an e-mail, have the user wait for a confirmation e-mail, ask for it again, wait again and then at some point find their mistake if you could have immediately prompted them fix their mistake as they put in user@gmailcom? Validation doesn't have to catch every mistake to be useful.

You can allow basically anything as an email address (exactly one @ symbol, text ahead and after, with at least one . after, something like /^([^@]+@[^@]+\.[^@]+)$/), and still verify that the user is human (captcha or something) and verify the email address (send signup email).

Not only would it suffice, it is the only way to tell if an email address is capable of receiving mail. There's no regex that can tell you that.

last Name Null - https://www.wired.com/2015/11/null/