Hello there, Mer.
Yes, I agree with Rustler, you can ask your customer to setup their emails and allow intuit emails to go to their inbox. Intuit uses quickbooks@notification.intuit.com as the default email address when sending sales transaction such as invoices.
Also, this article provides a list of legit emails from Intuit that you can allow: Is this email I received a legitimate communication from Intuit?.
Should you need more clarifications or other questions, just let me know and I'd be glad to further assist.
