The https://outlook.office.com/SMTP.SendAsApp scope is not valid for the Graph API, but for the Exchange Online REST API. Moreover, this is an application permission ("role"), not delegate one. Thus you need to use a confidential client (the client credentials flow). Here's an example:
GET https://login.microsoftonline.com/common/adminconsent/?client_id=12345678-1234-1234-1234-1234567890ab&scope=https://outlook.office.com/SMTP.SendAsApp
where the client ID must correspond to confidential client app with said role added under the "Office 365 Exchange Online" resource.