The Jargon File (version 4.4.7, 29 Dec 2003):

snail-mail
 n.

    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.)