Welcome to Meta ![]()
There is the pending users reminder delay minutes site setting.
That helps with how often the message is sent.
Sometimes it happens that not all users are automatically approved when approval for new users is enabled. I know of at least 2 bugs that cause this ([1] & [2]).
You can try to find and approve the users. I prefer to use the data explorer plugin, but it’s also possible to export all users in case you don’t have access to that plugin.
Then you can try to use the approve button on the admin page of those users. If the button doesn’t work, you’ll have to wait for a fix (Approval button on user's profile doesn't work)
