CYCBusiness1526....I see that you too are very frustrated by the issue that exist with customer email addresses not working correctly when using Projects or Customer sub-Accounts.  I know there are MANY others that have faced the same software failure and have cancelled their QuickBooks subscription to find another Accounting software platform that meets the basic expectations of its users. 

I also see that SECAMPBE has had this ongoing issue and I research this further, have found many others that have this issue but have not found this thread to complain about it.  

A new QuickBooks software update was made on 12/09/24 and the issue still persist and has not been fixed.

I believe our only options are to make as much noise as possible in hopes of getting this issue fixed OR that we all find a new software accounting platform for out businesses.

I have spent a lot of time & resources over the past 2 years in integrating my company with QuickBooks and leaving is not my first option BUT if  I and others choose this option, we should all let as many people know about the basic failures of this software so that others do not fall down this rabbit hole!