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.