onInsert.js

import("system.datetime");

/* In this process here basically nothing happens because everything is done within the UnlinkedMailMappingWrapper.
 * To make the UnlinkedMailMappingLink_entity NOT read only we've to specify some code (comments excluded). This seems (and IS) odd but it's currenty
 * the only way to do the job
 * If you wan't more information about the entity and why it exists like this: See the documentation of the entity.
 * 
 * Tl;dr; do not remove the following line: */
datetime.date();