When non-nil, the value post-delivery-function should be a function designator for a function of one argument:
The system calls post-delivery-function after delivery. successp is true if delivery was successful and false otherwise.
Note: during the delivery process, the Lisp system can be in an unstable state, so it is not always possible to recover when delivery is not successful.
LispWorks Delivery User Guide - 10 Aug 2017