I just changed a customer's email and the estimates are still being sent to the old email address. I've logged out and logged back in and this is still the issue.

The updated email address will only be applied to the newly created transactions, @Lynette.

I know how convenient would it be if the system will automatically update the details on the completed transaction. QuickBooks ensures that any changes made won’t affect your books.

If the customers change their contact information, especially their email addresses, it would be better to create an event after the modification. This way, the system will recognize the change and automatically applied it to the current transaction. For now, consider changing them manually.

You might want to visit this article about customizing invoices, estimates, and sales receipts in QuickBooks Online. This reference gives you complete information on how to create a more personalized template that able you to adjust the general layout and the email content.  Just select each section to view them. 

Don’t hesitate to reach out back if you have other concerns with our features. I’m always here to help you. Take care.