Dictionary.com
definitions

mail flag

noun Nautical.
  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.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018