Throughout the '90s there was no cooler record label other than Seattle based Sub Pop Records, home to Nirvana, Soundgarden, and Mudhoney. Their popularity, lead to them coming up with a subtle way of letting bands know their music didn't make the cut.
Two best things about this letter: the awesome way they addressed the sender with "Dear Loser"...
...and just in case bands didn't pick up on the fact that their music sounded like shit. It came with a clear this is "a rejection letter" label.