Just to add another data point, what I see in the message footer in mailing list mode** is this:
Visit Topic or reply to this email to respond.
You are receiving this because you enabled mailing list mode.
To unsubscribe from these emails, [click here]
** so I should be getting the same number of messages as you, excluding PMs.
We’re having operational issues with rate-limiting by some hosts of emails coming from one of our mail server IPs. To try to reduce these we’ve set the maximum daily emails from discourse. If the mailman-based mailing lists were still in regular usage we’d have to look at doing something similar with them.
There’s no ETA on when we’ll have the issues resolved.
I have now increased the limit from 100 → 500 emails per day, hope that is enough.
The default in discourse is 100, but previously we had overridden it to unlimited emails per day. As mentioned by Paul our out-going mail server was being rate-limited by a major email provider and I was trying to find a suitable option to reduce the volume of emails we were sending them.