[berkes]

You are missing the point. Sorry for that.

It doesn't matter if an email has a valid format: that says almost nothing about it's validity. The only way you can be sure an address can receive mail(today) is by sending mail to it. All the rest is theatre.

And all this only matters if the business cares about deliverability in the first place.

[bigtunacan]

No, I understood your point and I agree sending the email and getting some type of read receipt is necessary.

You seem to think that because of this client validation should be skipped. On that point I disagree. If you can tell that it's not a valid email address (bigtunacan@goog obviously invalid since missing a TLD) then no email should be sent. Good UX is to let the customer/user know there is a mistake in the email address.