Manual:$wgUserEmailUseReplyTo

Details
If $wgUserEmailUseReplyTo is set to true, user-to-user emails will contain the email address of the sending user in a Reply-To: header only, instead of directly in the From: line. Emails will appear to come from $wgPasswordSender, but replies will still be sent to the correct place.

It is clean-looking and convenient to place the sending user's email address in the From: line (this is the default behavior). However, there are several reasons why this may not be desired:


 * Putting the sending user's email address in the From: line doesn't as clearly differentiate user-to-user email (sent via the wiki) from directly sent messages (such as those sent from an e-mail client).
 * Another concern is that some mailers (like sSMTP) will use the From address as the SMTP envelope sender (the generator of the email message) as well. For open sites, this can cause email messages to be penalized for SPF violations (since the wiki server isn't authorized to send messages on behalf of other domains), as well as creating a privacy issue, as bounces containing the recipient's e-mail address may get sent to the sending user.