Dictionary
mail' flag"
Naut.
1. a flag of the International Code of Signals symbolizing the letter
Y, flown alone by a ship to indicate that it is carrying mail: a square flag with red and yellow diagonal stripes.
2. any of various other flags indicating that a ship is carrying mail.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.