mail boat vs packet boat

mail boat and packet boat both are nouns.

Word	Noun	Adjective	Verb	Adverb
mail boat	Yes	No	No	No
packet boat	Yes	No	No	No

As nouns, mail boat and packet boat are synonyms defined as:

Other synonyms of mail boat include mailboat, packet.

mail boat (noun)	packet boat (noun)
a boat for carrying mail	a boat for carrying mail