Email Verification Best Practices
More important than whether the email address exists is whether it actually belongs to the person who is signing up. Not all typos result in invalid addresses. Sometimes the address you get exists, but belongs to someone else.
If you’re sending marketing, the owner of the mistyped address didn’t actually ask for it, which makes it spam. And if you’re sending order confirmations, password reset codes, account statements, bills, etc., you really don’t want to send them to the wrong person.
The only way to be sure that the address you get belongs to the person who signed up is to send a confirmation email with a unique link and wait for them to click on it. (Even then, someone could click on it anyway for the lulz, but most people want to avoid unwanted mail.)