The UNKNOWN category is where the email address passes syntax and MX checks, but we can't say with a high degree of certainty whether the email address mailbox is valid or not. This can be because the recipient's mail server was temporarily down, or because their mail server always returns OK (catch-all).
Mail servers can be configured with a policy known as Catch-all. Catch-all domains basically accept any mailbox as valid when a verification check is performed, however the mailbox checked may actually be invalid. The only way to truly know if a mailbox is valid or not where a domain uses catch-all is to actually send an email. If it is invalid, you will receive a delayed bounce notification via email. As we do not actually send emails we are unable to do this.
Whilst our service can identify catch-all domains, it is up to you whether or not you want to include them in email campaigns.
We recommend introducing catch-alls gradually into your mailing list so that bounces are kept low. You can assume a similar proportion of invalid emails in the catch-alls to that of your validated emails. For example, if you had 10% Bad emails, you can assume that 10% of your Catch-alls are also potentially Bad.
The OK and BAD results will be very accurate and by emailing the OK file only you will find that your hard bounce rate is significantly reduced.