diplomatic pouch vs mailbag

diplomatic pouch and mailbag both are nouns.

Word	Noun	Adjective	Verb	Adverb
diplomatic pouch	Yes	No	No	No
mailbag	Yes	No	No	No

As nouns, mailbag is a hypernym of diplomatic pouch; that is, mailbag is a word with a broader meaning than diplomatic pouch:

diplomatic pouch: a mail pouch that is sealed shut and that is used to carry communications between a legation and its home office
mailbag: pouch used in the shipment of mail

Other hypernyms of diplomatic pouch include mail pouch.

diplomatic pouch (noun)	mailbag (noun)
a mail pouch that is sealed shut and that is used to carry communications between a legation and its home office	letter carrier's shoulder bag
	pouch used in the shipment of mail