Spam trap
A spam trap is an address that exists only to catch senders who did not collect their list properly. Mail to one is strong evidence of harvesting or list buying, and hitting them damages reputation quickly.
There are two kinds. Pristine traps were never used by a person and were published where only a scraper would find them, so mail to one proves the address was harvested. Recycled traps are real addresses that were abandoned and later reactivated as traps, so mail to one proves the list is old and never cleaned.
No verifier can identify a trap — a trap address is deliberately indistinguishable from a working mailbox, and that is the entire point. The only defences are avoiding indiscriminate harvesting and re-verifying old lists, which is to say the same defences as everything else.
Goes deeper: Why Your Cold Emails Land in Spam