"Snail mail penpals are those penpals that communicate with one another through the postal system, rather than on the internet which has become the more common medium." → https://en.wikipedia.org/wiki/Snail_mail

Snail mail is a slang term, referring to regular mail sent using a stamp. A snail, (Улитка), is very slow, and it rhymes with e-mail, so that is why this term is used!

"Snail mail" is the regular post! Even before the internet, the post was sometimes depicted as a cartoon snail delivering letters...