1.  A bag in which mailed matter is conveyed or shipped under public authority. [WordNet 1.5]

2.  A letter carrier's shoulder bag; as, in England they call a mailbag a postbag. [WordNet 1.5]

1st-class mail, air bag, air mail, bag, bag lady, bag net, bag of tricks, belt bag, body bag, book bag, bulk mail, burlap bag, burn bag, carrier bag, chain mail, clutch bag, coat of mail, dead mail, dime bag, direct mail, ditty bag, doggie bag, doggy bag, douche bag, drawstring bag, duffel bag, duffle bag, dust bag, electronic mail, evening bag, express mail, fan mail, feed bag, first-class mail, game bag, garment bag, gladstone bag, golf bag, grab bag, grocery bag, hate mail, hot-water bag, ice bag, in the bag, junk mail, kit bag, let the cat out of the bag, mail, mail boat, mail call, mail car, mail carrier, mail clerk, mail coach, mail fraud, mail order, mail order house, mail out, mail pouch, mail service, mail slot, mail train, mixed bag, old bag, overnight bag, paper bag, plastic bag, punch bag, punching bag, registered mail, ring mail, rosin bag, scum bag, shopping bag, shoulder bag, sick bag, sleeping bag, snail mail, sponge bag, surface mail, sweat bag, tea bag, telescope bag, third-class mail, toilet bag, tool bag, tote bag, traveling bag, vacuum bag, voice mail