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

These are the only two flags that are hoisted on British ships today, with the exception of the company's house flag, when they are entering port or passing at sea, and the mail flag on the foremast, which every steamship flies coming in to denote that she has mails on board.

From Project Gutenberg

Fourthly, we stated it to be highly desirable that the English Government should instruct their Commanders not to arrest steamers flying the German mail flag….

From Project Gutenberg