This is very frustrating.  I tried to send numerous invoices and kept getting a delivery error.  I just spent over 30 minutes on the phone with a customer service rep that had no idea there was a problem and very nice trying to help me on my end. He had no way to even reach out to engineering to see if it was on your end. I knew it wasn't my issue because I was sending to a couple different emails including my own.  QB's needs to tell the customer service reps when there is an issue.  Frustrated to say the least.