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.

Hello there, @greenspacemowing!

We haven't received any reports from other users who are having trouble sending invoices via email. You'll want to manually send the invoice via your personal email to check if this issue is related to your client's email.

From here, if you can send it without any issue, you'll want to contact your client's email provider to check if they're blocking all emails from QuickBooks. But if the issue persists, I'd recommend contacting our Customer Care Support. Someone can create a ticket ID associated with your account to further investigate the root cause of this sudden behavior.

1. Go to Help.
2. Select Contact Us.
3. Enter Invoice Bounced Back in the What can we help you with? field.
4. Click Let's talk.
5. Select Get a callback, Chat, or Call us.

Lastly, here's a few articles you can read for ideas about how you can manage your customer's and sales transactions: Income and Expenses for QuickBooks Online.

Keep me posted in the comments if you have any other questions. I'll be here to help.