It's a bit of a mess:
wgEmergencyContact, wgNoReplyAddress, wgPasswordSender and wgNotificationSender
# The wgPasswordSender and wgEmergencyContact address are configured by the installer
# wgEmergencyContact is only being used by ProofreadPage... WTF....
# This 'password sender' email address is used for almost everything: password emails, watchlist notifictions, EmailUser, LQT notifications, Echo notifications
# The noreplysender is used as the replyto address for watch list notifications (and AFTv5)
# wgPasswordSender is provided by the installer and defaults to false
# wgNoReplyAddress defaults to firstname.lastname@example.org (and probably no one configures it)
# wgNotificationSender is provided by Echo and defaults to wgPasswordSender
# email@example.com .... we should probably default that to false, since TLDs opened up.
# wgPasswordSenderName is removed, but still present in DefaultSettings
So as brion [[ https://www.mediawiki.org/wiki/Special:Code/MediaWiki/38375 | duly noted ]] in 2008: `we should probably refactor all this address crap`