postmistress vs adult female

postmistress and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
postmistress	Yes	No	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of postmistress; that is, adult female is a word with a broader meaning than postmistress:

Other hypernyms of postmistress include postmaster, woman.

postmistress (noun)	adult female (noun)
a woman postmaster	an adult female person (as opposed to a man)