e-mail has 3 distinct synonyms spread across 2 of its 2 senses. 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 one recorded opposite is snail mail.
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.
(computer science) a system of world-wide electronic communication in which a computer user can compose a message at one terminal that can be regenerated at the recipient's terminal when the recipient logs in
“you cannot send packages by electronic mail”
communicate electronically on the computer
“she e-mailed me the good news”
Synonyms, antonyms and the broader and narrower relations for e-mail come from Princeton University’s WordNet. Full sources and licences.