Definition from Wiktionary, the free dictionary
mailboat (plural mailboats)
- a boat used to transport mail
- 1922, James Joyce, Ulysses, Part I, 
- Stephen stood up and went over to the parapet. Leaning on it he looked down on the water and on the mailboat clearing the harbourmouth of Kingstown.
- 1938, Xavier Herbert, Capricornia, New York: D. Appleton-Century, 1943, Chapter VIII, p. 120, 
- […] the news and propaganda in imperialistic papers imported once a month by the mail-boat from the South lacked power because it lacked continuity and was stale.
- 1922, James Joyce, Ulysses, Part I,