The operating system supports a virtual
device, called a mailbox, that is used for communication between processes.
Mailboxes provide a controlled, synchronized method for processes
to exchange data. Although mailboxes transfer information much like
other I/O devices, they are not hardware devices. Rather, mailboxes
are a software-implemented way to perform read and write operations
For additional information about using mailboxes,
see HP OpenVMS Programming Concepts and the HP OpenVMS System Services Reference