I'm glad to see you posting here in QuickBooks Community. I'll provide different troubleshooting steps to make sure your customer will receive the invoices.
First, if you recently imported QuickBooks Desktop data into a QuickBooks Online company, you'll need to reset your email address. This is to ensure that the email you're using is updated.
Click the Gear ⚙️icon at the upper right corner.
Under Your Company, select Account and Settings.
From the Contact info section, review each email address and make changes as needed.
Hit Save and Done.
If the issue persists, I'd suggest asking your customers to check their junk mail and spam folders. If the email is still not found, you'll need to clear the Company email field and enter the desired email address. To see additional information and solution, you can click this article: 3 solutions for when customers aren't receiving your emails.