[thefreeman]

Nobody thinks a regular expression will prevent someone from signing up with an email that doesn't exist.

If you absolutely must have valid email addresses the only sure way is to make them confirm it with information from an email sent to the address.

[robomartin]

I think you are missing an important point. This isn't about email addresses that don't exist. You can't fix that. This is mostly about malformed or "illegal" addresses.

Scenario:

I somehow find myself at your landing page.

You have a single field for may email and a "Sign me up!" button.

I enter my email and sign up.

Your regex lies to you by thinking that the email is fine. In reality I made a mistake when I typed it in. I didn't notice it. Neither did your regex.

How are you going to contact me?

OK, if it is a small shop you can probably afford to have a human being review bounced emails and try to make some sense out of them. Well, what if you are signing up a thousand people a day?

Anyhow, the point is that a bad email addresses can cost you money both in customers that might never come back and also potentially in the manual work required to try to fix them manually.