mailboat vs packet boat

mailboat and packet boat both are nouns.

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

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

Other synonyms of mailboat include mail boat, packet.

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