The story
A retronym emphasizing postal mail's slowness compared to email. The phrase itself predates the invention of email, attested since in print as early as 1968, in an Advertising Council advertisement supporting the U.S. Post Office: "Help Stamp Out Snail Mail: Use ZIP Code." (High Fidelity 18:11 [November 1968], 40.)
Etymology from Wiktionary, CC BY-SA 4.0.