Let me tell you with a simple and basic explanation.
Step 1 — Syntax check — This is the very basic and core feature, the first step of validation for all email verifier services. Syntax check makes it possible to check whether an email address is spelled correctly — has no spaces, commas, and all the @s, dots, and check whether domain extensions are in the right place or not.
Step 2 — Domain check — Domain check is the second step of the email verification process. Domain check allows you to be sure the domain name of the email address is hosted actually exists, is registered, and working properly.
Step 3 — Email ping — This is the most sophisticated step of email verification. Email ping is the technical process of an email verification service pinging the exact email address with an EHLO message (to identify the domain name of the sending host to SMTP), then awaiting the response from the email server. The received server response will confirm the email address actually exists on the server and is active.
If you want to know more, I have a great blog on Email Checker you can visit for more deep knowledge.