When sending sales transactions like an estimate, QuickBooks Online (QBO) uses a default email “QuickBooks@notification.intuit.com. This will help update you on the status of the transaction sent. When the customer replies to any email sent using QBO, it will be directed to the Company's email that was set up on Company Settings.
To verify your company's email, here's how:
Go to the Gear icon.
Under Your Company, select Account and settings.
Choose the Advanced tab.
Under Messages, enter the correct email.
Click Save then Done.
The following articles will serve as your reference to learn more about creating and sending estimates in QBO: