I appreciate you for chiming in on this thread and bringing this to our attention, @LIS - AP.
I'm here to route you in the right direction for support to sort out this issue concerning your undelivered invoices in QuickBooks Online.
Beforehand, you'll want to let your customers check their settings and add *.intuit.com and *.intuit.net as trusted sites. The recipient's email server may be temporarily unavailable or blocking Intuit.
If the issue persists, I recommend contacting our QuickBooks Online Support team to have them further investigate this matter and have possible resolutions. Here's how:
- From the Search tab, click Contact Us.
- Enter your question and select Continue.
- Review and choose from the following: Have us call you, or Chat with us.
On top of that, I have an article that will help you account for your customer payments: Receive invoice payment.
Please let me know if you need clarification about invoices, or if there's anything else I can do for you. I'll be standing by for your response.
