After updating the "From" and "Reply To" field in the 'what will it contain' tab to a different email address, the email notification in sent mailbox has the "Email user label" from Email Account for SMTP and the overridden email address.

Example:

Email Account for SMTP has:

Email user label: IT Service Desk

From: <instance>@service-now.com

In the Notifications "What it will contain" tab, we have:

From: test@example.com

Reply-To: test@example.com

The resulting email will have:

From: IT Service Desk< test@example.com>

Reply-To: IT Service Desk< test@example.com>

This is because in the Email Account for SMTP you have the "Email user label" set as 'IT Service Desk'
and in the Notification you are overriding just the email with< test@example.com>

To override the label as well, please update the "From" and the Reply-To in the notification with the Label <email address>

In the example above it will be:

IT Service Desk <test@example.com>