snail mail has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 3 recorded opposites are electronic mail, e-mail, email.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
any mail that is physically delivered by the postal service
“email is much faster than snail mail”
Synonyms, antonyms and the broader and narrower relations for snail mail come from Princeton University’s WordNet. Full sources and licences.