I have confirmed emails with email exchange, done all of the steps to re-enter and re-save the email addresses and I am still receiving email delivery errors. These are customers that have received and paid multiple invoices in the past using the same info. These delivery errors are new and random with many customers that have been with me since January receiving a monthly invoice and suddenly started having this error with no changes to the customer info section. I have also confirmed that the customers email is not full, because they reply without error when sending them direct emails. Two most recent emails bouncing back is @aol.com, but it is also happening with others. 

Please have quickbooks team fix this, it is generating a lot of unnecessary/unwanted administrator work on my end that I am frustrated with.