Snail mail is slow post
Snail mail – named after the snail with its slow speed — refers to letters carried by conventional postal delivery services. The phrase refers to the lag-time between dispatch of a letter and its receipt, versus the virtually instantaneous dispatch and delivery of its electronic equivalent, e-mail.
It is also known, more neutrally, as paper mail, postal mail, land mail, or simply mail and post. Snail mail penfriends or penpals are those that communicate with one another through the postal system, rather than on the internet which has become the more common medium.
Source: Wikipedia