Problem: Client had accidentally set all User’s Receive System Emails to YES. Receive System Emails are for Super Users/Webmaster only – not for 1,000 members.
Solution: Use phpMyAdmin to select and confirm that all 1,000 Joomla Users whose Receive System Emails are set to sendMail=1 (1=Yes)
Then, update all Receive System Emails from 1 (Yes) to 0 (zero; No)
SET `sendEmail` = replace(sendEmail, ‘1’, ‘0’)
Note: swap out the “xxxx” for your database’s prefix_
Very helpful source: How to find who is receiving Joomla system emails – Dart Creations