Single client suddenly started bouncing email invoices back. Confirmed, reset address in QB (copied/pasted address from test email sent to me). Still bouncing back.

I appreciate you for performing the recommended troubleshooting steps to get this sorted out, greenspacemowing.

For bounced emails, there are two main reasons why it wasn’t sent. It's possible the email address was entered incorrectly. It could be there was a temporary issue with the recipient's email server.

One of the solutions provided is to clear and re-enter your email address. Since this is not working, you may need to ask an IT expert to have the QuickBooks Online mail server hostnames and IP addresses to be added to your outgoing mail servers.

Just follow the steps and details in Solution 3: Configure the server to accept QuickBooks Online mail server hostnames and IP addresses section in this article: 3 solutions for when customers aren't receiving your emails. 

If the same thing happens, I'd suggest contacting our Customer Support Team. They'll pull up your account in a secure environment and investigate what's causing this issue.

Here's how:

1. Click the Help icon on the top right. 
2. Tap the Contact Us button at the bottom. 
3. Enter a short description of your concern. 
4. Tick Continue. 
5. Choose Message an agent to connect with our support. 

I have articles here about why email bounces and invoices dropped:

• Undelivered Invoices
• How to view the list of bounced emails with delivery errors

Keep me posted if you need anything else in QBO. Wishing you and your business continued success. Take care always.