The trick is to first define what we mean by ‘valid’.
We are developers, we are technical folk, so it’s no surprise that the prevailing wisdom is to check that it matches the official criteria, some examples of the diversity of the official criteria are…If you have a well laid-out form with a label that says “email”, and the user enters an ‘@’ symbol somewhere, then it’s safe to say they understood that they were supposed to be entering an email address. Next, we want to do some validation to ascertain if they correctly entered right?
Then it tells you whether the email address is real or not.
Some mail servers do not co-operate in the process, in such cases, the result of this email verification tool may not be as accurate as expected.
Have a bad reputation, and you'll struggle to hit those inboxes.