This issue has been going on for a year now, with zero resolutions. I have even created a new customer and input the same credentials with the same undeliverable error. But I can email my customers just fine. The error is coming from QuickBooks. I need to be able to invoice my customers. There are three customers I am receiving this error for. All have a Yahoo handle. But I also have other customers with a Yahoo handle that receive my invoices. It is very frustrating, to say the least.  I have been paying for QBO for a year without a resolution. I have faithfully been using QuickBooks for a decade. We need this problem resolved. I have not heard from any engineers about a fix.