Thank you for bringing this to our attention, @Kayr. I'm here to share details about your customer's email concern
QuickBooks Online (QBO) doesn't apply changes to email addresses retroactively to already created invoices, which means any invoices created with an old email address will retain that email. This is why you need to manually update the email before resending those invoices.
For now, I recommend continuing to manually change the emails before sending them to your customers. Rest assured that the new email will reflect on your future invoices.
Additionally, you can refer to this article about personalizing your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Let us know if you have other concerns with your customers and invoices. We're here to help.