exim.pl version 15 tracks more info about each email being sent, including each sender address, which can be multiple, per message ID. The internal tracking of this data requires an array. The bug was using the old sorting mechanism, which adds a new entry to the end, then sorts the whole array. For huge arrays (eg: many email sends), this would mean one sort per email, which will slow down the server. The change is to use the newer/fasert insert method, which inserts an entry to it's correct spot, and shifts the array as needed. No sorting is done, making it much quicker.