How the email verification tool works
We check the email address format.
Then make sure the domain name is valid.
In the last step, it extracts the MX records from the domain records and connects to the email server (via SMTP and also simulates sending a message) to verify that the mailbox actually exists for that user/address. Some mail servers are not involved in the process, in such cases the result of this email verification tool may not be as accurate as expected.
An email validator is a simple little tool for validating an email address. It's free and pretty easy to use. Just enter your email address and click the verification button. It tells you if the email id is real or fake.
Sometimes it can't predict correctly and say unknown because some email providers have set some limits and limitations or they just don't like any kind of checking checks on their mailboxes.