Enter a clue or pattern or both:
The Clue
Undeliverable letters: Slang (5 letter answer)
The Answer
DEADS
Tags
Notes
The answer DEADS is so rare that it appears less than once every 2500 puzzles.
Definition
DEAD as a noun:
1. people who are no longer living; "they buried the dead"
2. a time when coldness (or some other quality associated with death) is intense; "the dead of winter"