[robomartin]

It's highly application dependent, right?

Say it's a signup form. Your validator lets a bad email through. The potential client/customer left. You can't email them to ask for correct email. They are gone. They may never come back (depends of what they were there for, right?). In that case I would want to do as much as might be reasonable in order to ensure that I capture someone who might eventually translate into revenue.

In other cases it might be just fine to store absolute gibberish and deal with it later. That said, if you have an incredibly popular site and are receiving thousands upon thousands of sign-ups per day, do you really want to store junk? Someone is going to have to go clean it up before it is of any use.

Anyhow, my main point, perhaps, is that one should understand what these magic regex email validators are and are not. That's all.

The reason I bring this up whenever relevant is because I have been bitten by this problem in the past. I only understood the problem when an existing customer informed me that they could not sign-up to receive info on a new product because my email validator was kicking them out. He just picked-up the phone and called me. I never did learn how many people I lost because of that damn regex I grabbed from an authoritative source on the 'net.

[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.