Posts in rapid succession do not send all emails

This forum must approve all posts. As a result some posts are published in rapid succession: when a moderator logs in they approve the posts quickly after each other.

Most categories are set to “watching” for all users and almost no users log in, the forum is relying very heavily on email notifications.

The problem is that whenever posts are published in rapid succession (less than 1.5 minutes apart), not all emails are being sent. This query shows the count of all sent emails (entries in email_logs per topic). The amount of users that should be getting an email is around 2250. When people have logged in recently (about 1%) they don’t get an email, so the number varies a bit but should be between 2200 and 2250.

As you can see in the first three lines, posts published quickly after each other do not have the full amount of emails.

What could be going on here?

Oh, max_emails_per_day_per_user is set to something really high.

columns: topic, date/time, category, # emails sent