I am getting delivery failures with my invoices, not all but some.

Hi there,

Welcome to the Community again. QuickBooks is designed to help make your work easier and it's important you get to experience this without running into errors. Don't worry, I'll be glad to point you in the right direction so you can continue to manage your work with peace of mind. 

In order to troubleshoot the issue you've described, I encourage you to check out this helpful article as it provides more info about: Delivery Errors when trying to email a sales form. The article goes over the variables that may be causing this issue and it also provides the solutions as well. 

Let me know if this info helps. I'll be here in case you need additional support. Otherwise, I wish you a great rest of the week.