Hi there, @MaryBethP,
It's nice to see you here in our forums. Let's make sure your client gets the invoices you sent them.
You can have your members check their email's junk, spam or bulk email folder and see if they can see it there. When you send invoices directly from QuickBooks Online, your customers will receive it from replyTo@intuit.com.
Another option is clearing and re-entering the email address. Here's how:
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 that doesn't work, I suggest contacting an IT personnel to add QuickBooks Online mail server host and IP addresses to your outgoing mail servers. I've added here a related article with the complete steps on how to configure your email settings: 3 solutions for when customers aren't receiving your emails
I want to make sure this issue is resolved. Please feel free to post an update below. I'm here to lend you a hand anytime. Have a great weekend!