Manual:$wgUserEmailUseReplyTo/en

Details
If  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, but replies will be sent to the email address of the user sending the user-to-user email.

It is clean-looking and convenient to place the sending user's email address in the From: line (this is the default behavior). There are several reasons why the default setting of false may not be desired and should be set to true. They are:


 * Putting the sending user's email address in the From: line doesn't as clearly differentiate user-to-user email (an email sent via the wiki) from directly sent messages (emails sent from an email 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). This behavior 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) e.g. anyuser@userdomain.com is not authorized to send mail on yourwiki.com domain.
 * MediaWiki installations on a shared server may fail to send user-to-user email due to spam prevention by disabling open mail relaying.

Setting to true may create a privacy issue, as bounces containing the recipient's email address may get returned to the server's sending user email address.