It's a bit of a mess:

We have:
wgEmergencyContact, wgNoReplyAddress, wgPasswordSender and wgNotificationSender

1. The wgPasswordSender and wgEmergencyContact address are configured by the installer
2. wgEmergencyContact is only being used by ProofreadPage... WTF....
3. This 'password sender' email address is used for almost everything: password emails, watchlist notifictions, EmailUser, LQT notifications, Echo notifications
4. The noreplysender is used as the replyto address for watch list notifications (and AFTv5)
5. wgPasswordSender is provided by the installer and defaults to false
6. wgNoReplyAddress defaults to reply@not.possible (and probably no one configures it)
7. wgNotificationSender is provided by Echo and defaults to wgPasswordSender
8. reply@not.possible .... we should probably default that to false, since TLDs opened up.
9. wgPasswordSenderName is removed, but still present in DefaultSettings

So as brion duly noted in 2008: we should probably refactor all this address crap