Instances of the system class
mailbox are used to communicate between processes. The communication is done by "sending" objects (any Lisp object) to a mailbox, and "reading" these objects from the mailbox. The objects will be read in the order in which they were sent. Sending is done by mailbox-send or mailbox-send-limited. Reading is done by mailbox-wait-for-event, mailbox-wait or mailbox-read. All mailbox access functions are thread-safe. You create a mailbox by using make-mailbox. You can also obtain the mailbox of a process by process-mailbox.
LispWorks User Guide and Reference Manual - 20 Sep 2017