Catch-all domain
A domain set to accept all email, even to nonexistent mailboxes, which prevents verifying an address via SMTP.
On a catch-all domain, the server answers "yes" to any address. So SMTP cannot tell whether first.last@ truly exists: the probe always returns a positive, inconclusive result.
Catch-all addresses are classified "uncertain" rather than valid. To qualify them you must combine other signals (the address appearing in sources, consistency with the observed format) instead of trusting the SMTP test alone.
Related terms
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