Welcome aboard to the Community, @panoramagardens.
The invoices coming from QuickBooks may be triggered by Junk mail filters. It happens when emails are coming in with similar type of information.
I suggest letting your customer log into the account and configure the settings to ensure it'll show directly in the inbox. If they need assistance performing the process, they'll need to reach out to the email provider's Customer Support Team.
I've also included an article in case your customers aren't receiving your emails: 3 solutions for when customers aren't receiving your emails.
I'm only a few clicks away if you have any other questions. Wishing you and your business continued success.