Integrate Domain Checking into form validations

[Rio517]

We wind up getting a failry high number of bad email addresses from users misspelling their email addresses domain names. About 5%. Right now, we use jquery.validate for general form validations, but it would be great if we could integrate domain name checking into our validation process.

Eample page: https://mayday.us/join/
Mail check repo: https://github.com/mailcheck/mailcheck