Mail servers can be configured with a policy known as Catch-all. Catch-all domains redirect all emails sent to non-existent mailboxes to a central email box for manual inspection. Catch-all configured servers cannot respond to requests for email address verification, as all mailboxes will return OK and will in theory accept mail.
Whilst our service can identify catch-all domains, it is up to the customer whether or not they want to include them on their email campaigns.