For most applications email validation is really not nesecarry. instead a service can rely on email verification. If the address is invalid, mails sent to it will never arrive at a destination.
It is good UX to let user immediately know if they enter a typo or an e-mail that is bound to fail. It is a real waste of time if user enters wrong e-mail and keeps refreshing inbox and spam for the following 5 minutes and still unsure afterwards whether they put wrong e-mail or the server just wasn't working correctly.
Also you will end up fucking your reputation by sending out emails to bad mailboxes all day long
Notably, this doesn't work for sending out purchase receipts.
One of the big problems we had at my old job. We were selling event tickets, the even we were selling for were sold in batches and usually sold out in minutes, so people would rush to purchase. We saw so many gnail.com addresses and people complained about not receiving their tickets