Snail mail is a slang term for the regular postal service (for example, the U.S. Postal Service) with the implication that it is a lot slower than E-mail . It's worth noting, perhaps, that in the early days of the Internet, it was proposed that the U. S. Post Office manage e-mail. Electronic messages would come to your local post office and then be delivered to you along with the regular mail. The proposal was not considered for very long.
Snail mail or not, one must acknowledge that regular postal services offer a number of valuable services not likely to be available soon on the World Wide Web.