I am trying to invoice my regular customers and it keeps telling me it does not recognize any of the emails for the customers.

Thanks for posting here in the Community space, Harris. I'll guide you on how to send the invoices successfully to your regular customers using QuickBooks Online (QBO).

Note that when sending invoices, you may encounter issues where your customers' emails are not recognized due to your account being inactive. To avoid this, please check if your account is currently active. If it's not, you can resubscribe to reactivate your QuickBooks. 

If your account is still active and has no issues, I recommend conducting browser troubleshooting steps to rule out any problems you encountered in QBO. Know that browsers accumulate temporary internet files that can affect your browsing experience. Too many internet files can sometimes change how your QBO company behaves, resulting in errors when performing tasks. See the following keyboard shortcuts for suitable browsers to open a private window:
 

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Safari: Command + Option + P

Check out this article for a step-by-step process on clearing your cache: Clear cache and cookies to fix issues when using QuickBooks Online.

On the other hand, you may also want to personalize your sales form in QBO to look organized and fit your business standards: Customize invoices, estimates, and sales receipts in QuickBooks Online.

If you have follow-up questions about managing invoices in QBO, let me know by leaving a comment below. I'm more than happy to help you again. Keep safe!