registered mail vs registered post

registered mail and registered post both are nouns.

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

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

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

registered mail (noun)	registered post (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