Manual:$wgUserEmailUseReplyTo/de-formal

Details
If  is set to, 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 may be clean-looking and convenient to place the sending user's email address in the From: line. But there are several reasons why setting of 'false' may not be desired. They are:

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.
 * 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).
 * MediaWiki installations on a shared server may fail to send user-to-user email due to spam prevention by disabling open mail relaying.