Hello there, @BQ backwards sucks.

Currently, we have an ongoing investigation about undeliverable or bounced e-mail error for invoices in QuickBooks Online (QBO). I assure you that our product engineers are actively working to have a permanent fix as soon as possible.

Even though you've received an invoice undeliverable message, you can visit the Audit Log or history page to view whether the said transactions were already received and viewed by your customers. Here's how:

1. Go to the Gear icon.
2. Select Audit log.
3. Use the fields on the Filter panel to choose the appropriate User, Date, or Events options to narrow the results.
4. Click Apply.

I've attached a screenshot below that shows the last two steps.

In line with this, I'd recommend contacting our Customer Care team. They can add your account to the list of affected users. This ensures you're in the loop about the investigation's status and its fix. You can provide the INV-52221 to our representative as your reference.

Additionally, you're able to email and print multiple invoices and other sales forms in QBO. This way, you're able to save time and effectively monitor your sales transactions. You can learn more about this through this article: Email or print multiple invoices and sales forms.

I'm just around if you have other concerns or follow-up inquiries about invoices and other sales transactions in QBO. Take care always, @BQ backwards sucks.