What is the origin of the idiom "snail mail" and when to use it?

The idiom “snail mail” emerged in the 1940s to refer to traditional postal delivery in contrast to the much faster electronic mail, which became widespread in the 1990s. This expression draws a humorous comparison between the slow pace of a snail and the perceived sluggishness of the postal system. It is used to refer to traditional postal delivery in contrast to the much faster electronic mail, which became widespread in the 1990s.