Manual:$wgPasswordSender

Details
Password reminder email address The address we should use as sender when a user is requesting his password. Also used as the password sender for other email notifications.

Usage in DefaultSettings.php
Note that from v1.5.0 upwards, the default value in DefaultSettings.php is initially set to the same value as was used in 1.4.x (with 'Wikipedia' changed to 'MediaWiki' in v1.6.0) but this is overridden later in DefaultSettings.php where it sets it to the value of $wgEmergencyContact, so this initial value is never seen (hence the current default value, above). This information is largely irrelevant, but is included here because it can cause confusion if you are browsing DefaultSettings.php to check the default value.