From https://hunter.io/email-verifier :-
* Valid format: First, we verify the format of the email address is correct and looks something like "email@provider.com".
* Gibberish Email Address: We check to ensure the address doesn't look like a random email address. For example, 1e49cs1pb6@company.com * doesn't pass the test.
* Disposable Email Address: We check if the email address has a domain name used for temporary email addresses.
* Webmail Email Addresses: We verify if the email address uses a webmail like Gmail or Yahoo.
* Presence of MX Records: We check if there are MX records on the domain. If there aren't, the email address can't receive emails.
* Presence of SMTP Server: This check is successful if we are able to connect to the SMTP server (indicated in MX records).
* SMTP Check: We test the email address and see if it bounces or not, without sending an email.
* Accept-all Domain: We check if the server has a catch-all policy which accepts all the email addresses.
For the "SMTP check", they will be doing something like this, programatically: https://www.labnol.org/software/verify-email-address/18220/