New User Notifications are Not Sending

Since the 4.5 WordPress update, once again (this is a 10 year old recurring off and on problem), new user notifications to admin are not sending. ALL Woo related emails work brilliantly and quite frankly, I can live without the admin notifications WordPress sends. However, it concerns me other problems may arise if we don’t solve this problem. Help?

P.S. I use Postman SMTP and the emails are NOT sending. This is not a hosting or PHP email issue I assure you. I’ve been troubleshooting this, along with so many others, for years. It works for a while and then just stops. This is happening on two of my sites: schoolofbookkeeping.com and 74systems.com. Both run the latest everything, tried and tested plugins and Woocommerce, Woo Subscriptions and Woo Memberships.