mailboat

postivene a boat used to transport mail
1922, (w), w:Ulysses (novel)|Ulysses, Part I, http://gutenberg.net.au/ebooks/fr100003.html

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.

(..) 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.