...if you know roughly where someone lives, but have lost their address, you can draw a sketch map on the envelope of your Christmas card to them...
That's what Peter O'Leary's former colleague did - and the Royal Mail managed to deliver the card in 9 days! Click on the picture for the BBC story.
Apparently, other people have sent mail just with an OS grid reference, and there is even a book by a woman who sent 130 letters with disguised addresses and puzzles for the postman on the envelopes... Of 130 letters, 120 of them reached their destinations!
The Royal Mail have a special warehouse for "undeliverable" mail, and 300 staff "leave no stone unturned" in their quest to get it to where it should be!