Manual:$wgUserEmailUseReplyTo

From MediaWiki.org

Email settings: $wgUserEmailUseReplyTo
Set to true to put the sending user's email in a Reply-To header instead of From. ($wgEmergencyContact will be used as From.)
Introduced in version:	1.12.0 (r30014)
Removed in version:	still in use
Allowed Values:	(boolean)
Default Value:	false

Other settings: Alphabetical | By Function

[edit] Details

Some mailers (eg sSMTP) set the SMTP envelope sender to the From value, which can cause problems with SPF validation and leak recipient addressses when bounces are sent to the sender.