Right now, there's an ongoing issue where some QuickBooks outgoing emails, like invoices, are blocked or marked as spam by recipients. That's why customers are receiving invoices, invoices are bouncing back/not being sent, or gets marked as spam.
Our Product Team is currently implementing measures to fix the issue. You can perform the workaround I've outlined below.
Save invoices as a PDF and send them via email through your business or other email address.
Send a customer the link to their invoice through SMS or messenger service. Simply click Save and share link, then select Copy link, and paste it to your SMS.
Save the invoice as a PDF, print it, and send their invoice via email.