mail has 11 distinct synonyms spread across 5 of its 7 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
the bags of letters and packages that are transported by the postal service
the system whereby messages are transmitted via the post office
“the mail handles billions of items every day”
a conveyance that transports the letters and packages that are conveyed by the postal system
any particular collection of letters or packages that is delivered
“your mail is on the table”
(Middle Ages) flexible armor made of interlinked metal rings
Synonyms, antonyms and the broader and narrower relations for mail come from Princeton University’s WordNet. Full sources and licences.