I am trying to retrive information about mails within a users mailbox. I am using the following snippet of code:
graphClient.Users[mailbox]
.Messages[messageId]
.Request()
.GetAsync()
.Result;
Which works fine for the most requests.
The messageId is read previously from the graph API.
The app registration has the following permissions.

Found this: using microsoft graph api to read mail behalf of users in the enterprise -> I'm using the application login method.