I’ve just discovered quite a few messages in a mail folder that I rarely check. They’re all replies to forum notification emails. If you look closely at notification emails they come from [email protected]. This is to avoid exposing the email address of forum users. So when you reply to a notification it’s sent to a deep dark hole email address that ends up in the folder I rarely check.
If you want to send a reply to a post that you’ve been notified about you need to log into the forum and reply in the forum - whether it’s a post or a PM.