Included in the day 1 mailing was an insert which announced the puzzle and ended with the pattern of letters
GR QRW WKURZ DZDB DQBWKLQJ ZH VHQG BRX
which, when decrypted using a Caesar cipher with a shift of +3, reads
DO NOT THROW AWAY ANYTHING WE SEND YOU
Due to the unambiguously important nature of this message, this theory is listed as proven.