Snail mail

Snail mail is a slang term describing traditional paper mail sent by postal service. As you likely know, it can take several days or even weeks to receive a message by post, whereas e-mail and other forms of modern digital communication are nearly instantaneous. Consequently, snail mail is aptly named as it is a much slower form of written communication by comparison.

Computer slang, E-mail, E-mail terms, Retronym