Hi there,
Sending invoice emails from QBO is one of the primary functions of the account, and makes it easy for you to get paid. I can appreciate that receiving an Undelivered status after trying to send your email isn't the outcome you'd expected. I'd be happy to explain what may be causing this, and a few steps you can take to get this resolved.
If you've sent emails to this address before without any issues, I'd recommend reaching out to the email provider to find out if there's any server issues on their end. If not, you should proceed to the following browser troubleshooting steps below.
Clear the cache and cookies in your web browser. These stored Internet files can build up over time and cause bugs in QBO and other sites. Here's how to clear them if you're not sure: How do I clear my browser cache and temporary Internet files?
If the issue persists, try using an incognito window in Google Chrome. That's a private window that runs independently of the main browser. Here's more information about what that is and how to use it: Browse in private. You can also try using an alternative browser.
If your emails are still showing as undelivered, please reach out to our tech support team.
Have a great day.