Let's double-check your email address for return to make sure it's correct. I'll show you how to configure or toggle your email address in the preferences to send transactions in QuickBooks Online (QBO).
To do that:
Go to the Gear icon, then Company Settings.
The Company tab will be selected by default. Select Edit to the right of the Contact Info section.
Enter your company's email address in the Company email field.
Otherwise, we can perform some basic troubleshooting steps to fix this. Let's try sending an invoice to a customer using a private browser or incognito window. This will eliminate any browser history that may cause a glitch inside QuickBooks. You can refer to the following keyboard shortcuts below:
Google Chrome: Ctrl + Shift + N
Microsoft Edge: Ctrl + Shift + P
Firefox: Ctrl + Shift + P
Safari: Command + Shift + N
When the customer responds with a correct return email, I suggest you close out the private window and clear the cache on the regular browser. This will remove previously-stored browsing data that might have caused the issue. Otherwise, you can try other supported browsers to help narrow down the cause of this behavior.
To help you with the future task and tips when using QBO, you might want to visit our Help Articles.
Get back to me if you need help with something else. I'd be happy to assist. Enjoy the rest of the day!