From <a href="https://hunter.io/email-verifier" rel="nofollow">https://hunter.io/email-verifier</a> :-<p>* Valid format: First, we verify the format of the email address is correct and looks something like "email@provider.com".<p>* 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.<p>* Disposable Email Address: We check if the email address has a domain name used for temporary email addresses.<p>* Webmail Email Addresses: We verify if the email address uses a webmail like Gmail or Yahoo.<p>* 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.<p>* Presence of SMTP Server: This check is successful if we are able to connect to the SMTP server (indicated in MX records).<p>* SMTP Check: We test the email address and see if it bounces or not, without sending an email.<p>* Accept-all Domain: We check if the server has a catch-all policy which accepts all the email addresses.<p>For the "SMTP check", they will be doing something like this, programatically: <a href="https://www.labnol.org/software/verify-email-address/18220/" rel="nofollow">https://www.labnol.org/software/verify-email-address/18220/</a>