Let's perform a few steps so the client will receive the invoices. To start, have them check if it was sent to their junk or spam folder. If the invoice is still missing, you'll have to configure your email setup.
Here's how:
Go to the Gear icon.
Under Your Company, select Account and settings.
Click the Company tab.
Select the edit pencil icon for Contact info.
Clear the Company email address field and enter the desired address, even if it shows as the correct one. Make sure not to use any extra characters or spaces before, within, or after the address, you enter.
Click Save, and then Done.
Next, send yourself a test transaction to test it. If you’re getting the same result, you’ll have to use a different email address that has a different domain. If the same thing happens, I'd suggest contacting an IT professional. They can help you add QuickBooks Online mail server hostnames and IP addresses to your outgoing mail servers.