mailer vs doer

mailer and doer both are nouns.

Word	Noun	Adjective	Verb	Adverb
mailer	Yes	No	No	No
doer	Yes	No	No	No

As nouns, doer is a hypernym of mailer; that is, doer is a word with a broader meaning than mailer:

Other hypernyms of mailer include actor, worker.

mailer (noun)	doer (noun)
a container for something to be mailed	a person who acts and gets things done
an advertisement that is sent by mail
a person who mails something