I have just noticed this: Handling bouncing e-mails - #166 by david
It is about the removal of the bounce_score_threshold_deactivate setting. I wonder whether that was a mistake. If the default setting was hard to reach then the answer was to lower it.
An unintended consequence of that removal for a large forum seems to be trying to send emails to an increasing number of invalid addresses regularly over the course of many years. Either that could lead to trouble with an external service (like Mailgun, which suppresses an address after a single permanent fail bounce) or IP reputation.
As it is, unless I’ve misunderstood, Discourse thinks it’s sending emails that Mailgun is just refusing to send because of its suppression list - and it is not possible to synchronise Discourse’s approach with Mailgun’s.