Catch-all domain

A catch-all domain — also called an accept-all domain — is configured so that its mail server says yes to every recipient address and decides what to do with the message afterwards. Mail addressed to a mailbox that was never created is accepted at the door and then delivered to a shared inbox, silently discarded, or bounced hours later.

This is the single most important case to understand in verification, because it is the one where the SMTP conversation stops being informative. The check runs exactly as it would anywhere else: the domain resolves, the mail server answers, the recipient is accepted. What is different is that the acceptance carries no information. The same yes comes back for an address that exists and one that does not.

Reporting that as deliverable would be a guess wearing the clothes of a fact. A catch-all address is risky: the domain is real, the route is real, and whether the individual mailbox exists is not knowable from outside without sending a message — which verification does not do.

What to do with a risky verdict depends on what the address is for. For a transactional message to somebody who has just typed their address into your form, a catch-all is usually fine. For a cold list where the address has never engaged, it is the group most likely to contain addresses that quietly go nowhere, and the group where a bounce, if it arrives at all, arrives late enough to have already cost you sending reputation.