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 $wgPasswordSender, but replies will be sent to the email address of the user sending the user-to-user email.
Document about SparkPost Reply_to api can be found here: https://developers.sparkpost.com/api/transmissions/#transmissions-post-send-inline-content
BTW, their is another setting related to reply-to function too. https://www.mediawiki.org/wiki/Manual:$wgNoReplyAddress