Hello, usermledet.
I have information regarding what email address is used when sending forms.
When sending forms, the system will always use the company email (customer-facing email). We're unable to switch or change it unless edited from the Company settings.
As a workaround, you can download the estimate and send it using your user's email provider.
To download an estimate:
- Go to Sales, then click the Customers tab.
- Select a customer.
- Find the estimate you want to send.
- Under the ACTIONS column, click the arrow icon.
- Choose Print.
- On the preview, click the download button (see the screenshot below).

Though, I can see the benefit of having the system use the user's email address when sending forms. Not only this would reduce clutter from a single email inbox but this can help organize your user's workflow.
We periodically add updates to the program and the option to use the user's email address might be added in the future. I suggest checking our QuickBooks Blog for release notes on the new features and updates from time to time.
If you need help with the progress invoicing feature, you can check this article for a guide: Set up and send progress invoices in QuickBooks Online.
I'd be glad to help out again and if you need details about other features in QuickBooks Online. Always ready to assist.