Show a small CTA (just text, no button) on the new reply email to prompt an author to mark a reply as a solution. With the following copy:
Make sure to mark the correct reply as the solution when your question gets answered.
On this email: https://github.com/laravelio/laravel.io/blob/main/resources/views/emails/new_reply.blade.php
Should only be shown if the receiver of the email is the thread author.