App won't send my invoices to saved clients email addresses anymore, saying delivery error constantly. Absolute nightmare. Can some one help?

Hi there, @bgconservation.

I appreciate you bringing this to our attention. I'm here to share some information regarding your concern about sending invoices in QuickBooks Online (QBO).

Currently, there’s an open investigation regarding this matter. Thus, our product engineers are working on releasing a permanent resolution to this issue.

In addition, so you’ll be added to the affected user’s lists, I’d recommend contacting our Customer Care Team. Rest assured, once an update is released about this ongoing investigation, you’ll be notified via email. 

Here’s how to get in touch with them:

1. Click the Help icon located in the upper right-hand corner.
2. In the Search window, click the Contact Us button located at the bottom.
3. Enter your question and click Let's talk.
4. Choose either Chat with us or Have us call you.

Please make sure to review their support hours to know when agents are available. This way, you can contact them at a time that is convenient for you.

For future reference, I'm adding this helpful article about recording invoice payments manually in QuickBooks Online: Record invoice payments in QuickBooks Online.

You're always welcome to post a reply on this thread if you have more questions or need help with other features. The Community is available to help.