I’d forgotten about that. I’m not sure that the bounce_score_threshold_deactivate setting was working to prevent Discourse from attempting to send emails to invalid addresses though. The problem is that once a user reaches the bounce score threshold, Discourse will stop sending them emails until the time period set by the reset bounce score after days setting has passed. At that time, the user’s bounce score will be reset and the process will start over again.
I’m not sure what the best solution for this would be. If I’m understanding things correctly, it does seem that over time a Discourse site will be attempting to send emails to an increasing number of invalid addresses.