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.

Example Sentences

"This is a United States mail boat," and he pointed to the mail flag flying at the stern.

From Project Gutenberg