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.
(Note: Actual garden snails progress at about 10 meters per hour, which is about 25-50 times slower than the U.K.'s Royal Mail; comparable measurements for other countries have not yet been made. More biologically apt terms might be “sloth-mail” at 250 m/hr or “tortoise-mail” at 270 m/hr. See http://www.newscientist.com/lastword/answers/789communication.jsp?tp=communication for details.)