My invoices are being received by customers but my system flags them as "undelivered"

I appreciate you letting us know what's happening and you reaching out to the Community, @Clive26.

Currently, there's an ongoing investigation regarding the invoices that are successfully sent but still showing undelivered status. Our Product Engineers are working on releasing a permanent resolution about this issue.

In the meantime, I'd highly suggest contacting our Support Team and provide this investigation number INV-54567 so they can add your company details to the list of affected users. By doing this, we will be able to send you an email notification once an update is available.

Here's how to reach them:

1. Sign in to your QuickBooks Online company.
2. Select Help (?) at the top right.
3. Select Contact Us.
4. Enter your query and then click Let's talk.
5. Choose to Start a chat

For their work schedule, you can reach out to them from 6 AM-6 PM PT Monday-Friday.

Just in case you wanted to create recurring transactions like expenses and invoices, you can refer to this article: Create recurring transactions in QuickBooks Online.

We appreciate your patience while this is being worked on. Just visit us again if you need anything else. Keep safe!