Hello there,
Welcome aboard to the Community space. I can clarify things about sending an invoice in QuickBooks Online.
An indicator or warning when entering an incorrect email address is unavailable. However, you have the ability to review the email attached to your invoice to verify the information (see screenshot below).

Once you create your sales forms, you can email or print them now or later. Check out this link to learn more about how to email or print multiple sales forms now or later: Email or print multiple sales forms in QuickBooks Online.
Drop a response anytime if you have additional questions about emailing invoices in QuickBooks. We're always ready to lend a hand. Take care always.