Tests whether a mailbox is full.
Because of multiprocessing, it cannot be guaranteed that a subsequent call to mailbox-send will work without expansion even if
mailbox-full-p returns false.
mailbox-full-p is intended to be used as a wait-function (or inside a wait-function), and once it returns false you should use mailbox-send-limited to actually send and check what it returns.
LispWorks User Guide and Reference Manual - 20 Sep 2017