I recently started using this email verification tool https://emailverification.whoisx... and I’m quite satisfied so far. I’ve set my website up to alert new users if they type in an email address that doesn’t use the correct syntax or doesn’t have an existing mail server and mailbox. That way email addresses that make their way into my mailing list are valid and existing. And more importantly, email verification helps ensure that clients can receive my emails. Aside from using the API, I also sometimes use the lookup tool to manually check if new clients enter the correct and existing email addresses.