Xenforo's built in email validator is very basic.
There is a service that offers paid API to check whether an e-mail is valid or not (bounced).

One of the service that i know is Kickbox: Email Address Verification Service
(free trial account is available)

There is also more simple approach than kickbox, but still better than xenforo's built in validator.
It is free.

REST API with cURL : (i think the result even can be cached by DB Optimise)
Email Validation — Mailgun API documentation

Simple javascript :
GitHub - mailgun/validator-demo: Mailgun email address jquery validation plugin http://mailgun.github.io/validator-demo/

This will be useful for forum with busy email sending, such as lot of new user registration, thus lowering mail bounce rate and make the forum admin happy