Sometimes emailed sales invoices say they have been sent but don't actually arrive. I have of course checked the email address and it is correct?

I’m delighted to have you here in the Community, @bjm4. I’ll assist you with your concern about undelivered invoices in QuickBooks Online (QBO).

Since you mentioned above, it can be seen as sent on your end yet didn’t actually arrive at your customer’s email. I suggest you have them check their spam folder. Also, to have this issue verified, you can also send the email to your personal email.

If you’re able to receive the email, and your customers are not. You can consult your IT professional to have your email set-up checked.

Let me know if you need help with your sales transactions. The Community always has your back. Stay safe!