WordNet (r) 3.0 (2006):

snail mail
    n 1: any mail that is physically delivered by the postal
         service; "email is much faster than snail mail" [ant:
         e-mail, electronic mail, email]

The Free On-line Dictionary of Computing (30 December 2018):

snail mail
papermail
paper-net

    (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)