The trouble with validating using SMTP is that it is fairly likely to introduce a delay. I still think it is better to just check the DNS, with a very low timeout and then send the email as long as there wasn't a negative response.
You can let the user immediately know that the email was sent, but then you can also push updates to the user whilst they're still on the site if there is a rejection/bounce.
I wrote https://emailprivacytester.com/, which does the DNS checks that I mentioned when you enter your email address, and then keeps you informed of the status of the message delivery as it happens, including any SMTP rejection messages.
You can let the user immediately know that the email was sent, but then you can also push updates to the user whilst they're still on the site if there is a rejection/bounce.
I wrote https://emailprivacytester.com/, which does the DNS checks that I mentioned when you enter your email address, and then keeps you informed of the status of the message delivery as it happens, including any SMTP rejection messages.