Email checker proceeds with several checks to determine if an email address is valid:
The email address must be well formed (e.g. {firstname.lastname}@{domain}).
It should not be generated by a disposable email service (such as mailinator.com).
The MX records should be present on DNS.
An SMTP server should run for the domain.
A mailbox with that email address should exists on the SMTP server.
Depending on the provided email address, we might apply extra-checks on a per basis.