registered post vs registered mail

registered post and registered mail both are nouns.

Word	Noun	Adjective	Verb	Adverb
registered post	Yes	No	No	No
registered mail	Yes	No	No	No

As nouns, registered post and registered mail are synonyms defined as:

registered post and registered mail: mail that is registered by the post office when sent in order to assure safe delivery

registered post (noun)	registered mail (noun)
mail that is registered by the post office when sent in order to assure safe delivery	mail that is registered by the post office when sent in order to assure safe delivery