What does Boxing Day have to do with boxing?
1590s, "assembly of people wearing masks and disguises," from French mascarade or Spanish mascarada "masked party or dance," from Italian mascarata "a ball at which masks are worn," variant of mascherata "masquerade," from maschera (see mask (n.)). Figurative sense of "false outward show" is from 1670s.
1650s, from masquerade (n.). Related: Masqueraded; masquerading.
This is good practise because external users do not need to know about internal changes in message routing. The external mail gateway needs to know how to route incoming replies back to the original sender.