Use an appropriate name in the Reply-To header

Use "Site Name <>" for the Reply-To header when the reply is to the site or a public topic.

Use "username <>" for the Reply-To header only when the reply is to a private message topic.

app/mailers/user_notifications.rb

@@ -215,6 +215,7 @@ class UserNotifications < ActionMailer::Base
       username: from_alias,
       add_unsubscribe_link: true,
       allow_reply_by_email: allow_reply_by_email,
+      private_reply: post.topic.private_message?,
       include_respond_instructions: !user.suspended?,
       template: template,
       html_override: html,