Catch-All domains always return a valid response from the SMTP service, whether the email address is actually valid or invalid. Some mail servers do this because they believe it will prevent people from harvesting real email addresses from their SMTP Server.
Some mail operators set up a catch-all address. For instance, if you mail email@example.com, the mail administrator will forward that email to a "catch-all" address, like "firstname.lastname@example.org". So they can look at all the emails sent to their mail server to see if anything important was sent to an invalid recipient.
We have found that catch-all domains get a lot more spam, because mail services have no way of telling if the email address is good or bad before sending. So spam operators just send to them, thinking someone will see the email.
Most Email Validation services will just return Valid for catch-all domains without telling you.
ZeroBounce.net identifies these domains for you. Since we are not able to validate them, it's up to you to decide if you want to take the risk to send to these domains. We do recommend that, if you decide to send emails to catch all domains, you segment these into a different group in your mailing platform.
Now, there are 3 behaviors of catch-all email domains:
1) An non-real email address is funneled into a catch-all inbox (This does not hurt your email reputation)
2) The non-real email is bounced back, saying the user doesn't exist (This does hurt your email reputation)
3) The recipient is actually valid and the email gets delivered