This means that the domain associated with the email is "accept all" and Hunter cannot verify if the email is currently valid.
Accept all domains are domains that are set up to accept all emails and, due to this, we trigger an alert in the verification section.
For example, the domain “eight25media.com” is one of these types of domains. So, if we check firstname.lastname@example.org we will always receive an “OK” response for any mailbox. This is why we cannot be sure the emails associated with them are valid.