What is a catch-all domain?

Some organisations configure their mail server to accept every address at the domain rather than reject unknown ones. Often this is to avoid losing mail to a typo. The effect for anybody verifying an address is that the server answers yes regardless, so the check proves nothing about that mailbox.

These domains are common enough to matter, and they are not evenly distributed: they cluster in particular industries and in companies running certain mail setups, which means a list concentrated in one segment can be mostly catch-all.

The honest handling is to mark the address as unconfirmed and let the person decide. Sending to it is a judgement about risk, not a mistake, and it is a judgement somebody can only make if the tool admits what it does not know.