If you tried to reply to a post or private message via email in the past four months, unfortunately, your reply never made it to the site. There was an issue with this feature due to a change I had made back in November that went unnoticed and it wasn’t reported to me until this past week by @Mark_Seaver (thank you Mark!) . Sadly those replies are lost in the system. You can search your email sent folder for npn.replies and this should show you what topics you responded to that never made it. Looking through the logs there were potentially hundreds of responses that never made it, which is heartbreaking.
I apologize profusely for this problem, it kills me that these posts were lost and I can only imagine how much better engagement would have been if it had been working.
Anyways, the feature is now fixed and you can reply to posts via email again.