Thank you for your suggestion, however, the article to which you refer says,
“To find out what email notifications are set as the default on your site, go to Settings > Discussion in the Dashboard. You should see a couple of options for when emails should be sent out, including when someone posts a comment and when a comment is held for moderation.
If these items are checked, an email notification will be sent to the author of the post on which the comment was made.”
I have done just that — for each user. both items are checked, yet no emails are being sent to two of the users, including me — and I’m an administrator.
