The Collaborative International Dictionary of English v.0.48:
mailboat \mail"boat`\, mail boat \mail" boat`\n. 1. a boat that carries the mail. Syn: mail boat, packet, packet boat. [WordNet 1.5]WordNet (r) 3.0 (2006):
mail boat n 1: a boat for carrying mail [syn: mailboat, mail boat, packet, packet boat]