Spam traps come in a few forms: pristine traps that were never a real inbox and exist purely to catch harvested or purchased lists, and recycled traps that were once real addresses, abandoned, and later reactivated as traps after a dormancy period.
Hitting even a small number of spam traps is a strong negative signal, because it directly indicates a list wasn't built through genuine opt-in, or wasn't pruned of long-dormant addresses. Traps are, by design, addresses that should never have received mail in the first place.
The defense is list hygiene: confirmed opt-in at acquisition, and suppressing addresses that show no engagement (opens, clicks) over an extended period rather than mailing a list indefinitely regardless of activity.