postbag vs mailbag

postbag and mailbag both are nouns.

Word	Noun	Adjective	Verb	Adverb
postbag	Yes	No	No	No
mailbag	Yes	No	No	No

As nouns, postbag and mailbag are synonyms defined as:

postbag (noun)	mailbag (noun)
letter carrier's shoulder bag	letter carrier's shoulder bag
	pouch used in the shipment of mail