Outbound Glossary
Catch-all
A domain that accepts mail to every address, so verification cannot tell a real colleague from a typo.
On a catch-all domain the server says yes to anything, which means the usual check of asking whether an address exists returns a useless answer. Verification tools mark these as risky or unknown rather than valid.
They are common at larger companies, so excluding them all throws away real prospects. The workable middle is to send to catch-alls in small, separate batches with their own mailbox, so if the guesses are wrong the bounces do not contaminate the rest of the campaign.