Catch-all domain
A mail domain that accepts every address, so a positive SMTP answer proves nothing about a specific mailbox.
A catch-all (also called accept-all) domain is configured to accept mail for any address at that domain, including ones that were never created. Ask the mail server whether [email protected] exists and it says yes, because saying yes to everything is the policy.
This matters because SMTP verification works by asking the receiving server about a specific recipient. On a normal domain the answer separates real mailboxes from typos and dead accounts. On a catch-all domain the answer is always yes, so the check cannot distinguish them.
The honest response is to report the result as unknown rather than valid, which is what our engine does. Sending to an unverified catch-all address is a judgement call: the address may be fine, or it may silently route to a black hole that never bounces and never reads.