The Collaborative International Dictionary of English v.0.48:

mailbag \mail"bag`\, mail bag \mail bag\n.
   A bag in which mailed matter is conveyed or shipped under
   public authority.

   Syn: mail pouch.
        [WordNet 1.5]

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

   Syn: postbag.
        [WordNet 1.5] mailboat