We perform a real time communication with the mail server to confirm the email address exists.
Example, if the email is firstname.lastname@example.org, we communicate directly with Gmail to confirm that the email address exists. Same thing goes for business emails. If the email is email@example.com, we communicate and confirm directly with Home Depot’s mail server. We never email or actually send any form of communication to the email address.
For consumer email addresses, we communicate directly with the service providers.
For business email addresses there are a few steps to the process.