Source : WordNet®

snail mail
     n : any mail that is physically delivered by the postal service;
         "email is much faster than snail mail" [ant: {electronic
         mail}, {electronic mail}, {electronic mail}]

Source : Free On-Line Dictionary of Computing

snail mail
     
         (Or "snailmail", "smail" from "US Mail" via
        "USnail"; "paper mail").  Bits of {dead tree} sent via the
        postal service as opposed to {electronic mail}.  One's postal
        address is, correspondingly, a "snail (mail) address".  There
        have even been parody USnail posters and stamps made.
     
        The variant "paper-net" is a hackish way of referring to the
        postal service, comparing it to a very slow, low-reliability
        {network}.  {Sig blocks} sometimes include a "Paper-Net:"
        header just before the sender's postal address; common
        variants of this are "Papernet" and "P-Net".  Note that the
        standard {netiquette} guidelines discourage this practice as a
        waste of bandwidth, since netters are quite unlikely to
        casually use postal addresses and if they really wanted your
        {snail mail} address they could always ask for it by e-mail.
     
        Compare {voice-net}, {sneakernet}, {P-mail}.
     
        (1995-01-31)