Paper mail, as opposed to electronic.
Sometimes written as the single word `SnailMail'. One's postal
address is, correspondingly, a `snail address'. Derives from
earlier coinage `USnail' (from `U.S. Mail'), for which
there have even been parody posters and stamps made. Also (less
commonly) called `P-mail', from `paper mail' or `physical mail'.
Oppose email.