We don't have synonyms for mail boat yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are sailboat, mailboat and sail boat.
(N) :
Mail boats or postal boats are a boat or ship used for the delivery of mail, and sometimes transportation of goods, people and vehicles, in communities where bodies of water commonly separate settlements, towns or cities, often where bridges are not available.
(N) :
Mail boats or postal boats are a boat or ship used for the delivery of mail, and sometimes transportation of goods, people and vehicles, in communities where bodies of water commonly separate settlements, towns or cities, often where bridges are not available.