[narag]

My point is that it doesn't matter if some arbitrary string looks like an email address, you need to check.

If it isn't valid the server won't annoy anyone. The problem is that the address is valid. And not theirs, it's mine.

The moment the users need to be careful, they will. Make the problem theirs, not mine.

"Sorry sir, the address you provided returns error" or "haven't you received the confirmation email YET? really? there are other customers in the line" and see how soon they remember the right address, perfectly spelled.

Even big ass companies like Paypal that have no problem freezing your monies, allow their customers to provide unchecked email addresses and send income reports there. (here)

[RussianCow]

You can (and should) definitely do both. But needing to validate that a user has access to the entered email address doesn't mean you should do away with client-side validation entirely.