Generally speaking, an email verifier works in at least three steps to get the job done, starting with whether or not the standard rules write out an email address. Then, it checks that the domain is active and can accept messages. Finally, this tool checks whether there is a live mailbox connected with an email address. This tool only flags the email to be removed from your list if, even at the steps mentioned above, anything is found to be invalid to save resources by not wasting them on addresses that couldn't receive emails.