One of my customers are now not receiving their emailed invoices and I get 3 emails saying it’s sent

Hello Waterselectrical, Does the customer have Intuit in their safe senders? It may be that the email provider is filtering as not known. If you can ask them in their email settings to add the following to the safe sender's list.

• uk_qbo_ops_support@intuit.com
• intuit@notifications.intuit.com
• quickbooks@notification.intuit.com
• *@*intuit.com