The term 'mail boat' refers to a vessel used to transport mail, usually between two ports. The two crossword solutions for this term are 'aviso' (5 letters) and 'PACKET' (6 letters). An aviso is a type of boat used by the French navy to carry dispatches and messages. A PACKET is a type of boat used to transport mail, passengers, and cargo between two ports. Both avisos and packets were commonly used as mail boats in the 19th century.