I'm here to provide some insights about the domain email when sending estimates via the QuickBooks Online.
When customer replies to any email sent using the QuickBooks Online, it will be directed to the Company's email or Customer facing email that was set up on Comp[any Settings (see screenshot below).
The notification.intuit.com is not a link, that is a domain email address of Intuit that is only used for sending (For intuit clients)/ Receiving (For client's customer). Make sure to ask your customer to hit Reply on the email (estimate) that they received, because the response will be directed on the Company's email, not the Intuit email.
If you need further assistance, feel free to reach out to our QuickBooks Online Support. They have the full access to pull up the account and review the email in a secure manner.