[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.