The notifications appear because accounts created from the website seem to have the "notify me of replies by default" option turned on. I didn't notice this because I signed up via the forums, which default to having the option off when they create a profile.
Threads that you've already posted in will continue to notify you of replies unless you go back and turn the option off manually. All the "notify by default" option in the profile does is check the "notify me of replies" box in threads for you, so those will stay selected until you undo them.
Basically: the main website code may need changing around to fix this, and all existing profiles may have this problem - if they do I can run a command on the MySQL database to set everyone
to not receive replies.
As for the mail debug message, debug mode is off, so you shouldn't even be seeing the message, and the mail server appears to be configured (it has no problem sending out email), so it's a tricky one. If anyone sees a debug notice could they screenshot it and send it to me?