The Collaborative International Dictionary of English v.0.48:

mailbox \mail"box`\ n.
   1. A public box for deposit of mail, where it is later picked
      up by the postal authority for delivery.

   Syn: postbox, letter box.
        [WordNet 1.5]

   2. A private box to recieve delivery of mail. The term is
      used both for boxes receiving mail delivered by the public
      postal authority, or by a private services, such as for
      mail to employees in large corporations.

   Syn: letter box.
        [WordNet 1.5 +PJC]

   3. (Computers) A location within a computer storage device
      where electronic mail is held until it is retrieved by the
      addressee. Creation and use of an electronic mailbox
      requires special software as well as a data storage
      device.
      [PJC]

WordNet (r) 3.0 (2006):

mailbox
    n 1: a private box for delivery of mail [syn: mailbox, letter
         box]
    2: public box for deposit of mail [syn: postbox, mailbox,
       letter box]

The Free On-line Dictionary of Computing (30 December 2018):

mailbox

   1.  A file belonging to a particular user on a
   particular computer in which received electronic mail
   messages are stored ready for the user to read them.  A
   mailbox may be just an electronic mail address to which
   messages are sent and may not actually correspond to a file if
   the messages are processed automatically, e.g. a mail server
   or mailing list.

   2.  A destination for interprocess messages in a
   message passing system.  A mailbox is a message queue,
   usually stored in the memory of the processor on which the
   receiving process is running.  Primitives are provided for
   sending a message to a named mailbox and for reading messages
   from a mailbox.

   (1994-10-20)