Hello there, multigraphic.
I appreciate you coming to the Community with your concern about invoices showing as undeliverable. I'd be glad to provide some info on this topic.
Since you verified that the email address was been entered correctly, there may be a temporary issue with the recipients email server. This article provides more information on why you may be getting the "undelivered" status: https://quickbooks.intuit.com/community/Income-and-expenses/Undelivered-Invoices-in-QuickBooks-Onlin....
Just to rule out the possibility of this being a cache error, I recommend clearing it. Clearing the cache will allow you to work with a clean slate and resolve performance errors within Quickbooks Online. This write-up will walk you through the steps of clearing the cache, depending upon which browser you're using: https://quickbooks.intuit.com/community/Getting-Started/How-do-I-clear-my-browser-cache-and-temporar....
Feel free to leave a comment letting me know how things go after giving this a try. I'll keep an eye out for your reply.
