Reply-by-email is a feature you can enable (or may already be enabled depending on your hosting) that allows people to reply to the notification email and it will be magically linked up and placed in the correct topic/pm when it arrives at your forum (I misspoke in the previous post, it’s not a special link they press, the magic happens behind the scenes).
You get a footer like this in the email:
There’s a bit more info here in case you haven’t got it set up yet:
(Though I have the mail receiver version - Configure direct-delivery incoming email for self-hosted sites with Mail-Receiver)