Not 100% sure, but I expect it asks the server if mail to an address is deliverable. I recently was doing some testing directly talking SMTP to a server and rec

Not 100% sure, but I expect it asks the server if mail to an address is deliverable. I recently was doing some testing directly talking SMTP to a server and recall a special check command. IIRC it said it check if the server would accept mail for an address. So I don’t think it would fail on a catch all. Assuming this uses that technique.