Discourse has a feature where it can periodically send you an email telling you a few highlights of the forum you may have missed. By default Discourse will do that if you haven’t visited the forum in a week.
Well when we switched to Discourse, I hadn’t thought through all the implications of this, because we have thousands of users who haven’t visited in the past week (mostly accounts from bots). And the forum did indeed try to sent emails to each of them, causing a lot of bouncing emails, slowing down the forum, and using up half of our monthly email quota. Because we need that quota for our regular notification emails (for replies, PMs, etc), I just disabled the digest feature until we could come up with a better solution.
That solution was to disable digest emails for everyone who hadn’t visited the forum in the last 30 days, and enable it for everyone who did. Unfortunately I couldn’t detect who had manually changed it themselves to preserve their settings. But you can now manually enable or disable it if you like. You can also change the frequency of digest emails.