Hi NikkiB7164,

Yes, we've seen other users reporting the same thing, and thank you for also alerting us to the Undeliverable Error Message issue within QuickBooks Online. Your feedback is useful and is currently guiding our Product Engineering team's efforts to resolve the problem comprehensively. Rest assured, we are committed to resolve this matter for you.

Please contact our QBO Support so they can add your account to ticket #INV-91619 for immediate updates straight from our engineers. You can follow ZackE's steps above.

For the time being, we have a helpful workaround for you: you can download PDF invoices and attach them to your emails when sending to customers or recipients. Your patience during this investigation and resolution process is greatly appreciated.

Once again, thank you for your continued support and understanding. Should you have any further questions or require additional assistance, please don't hesitate to reach back out through this thread. Your satisfaction remains our top priority, and we are here to assist you at every turn.