Access Denied Error When Using App Authentication for Shared Mailbox

Hello,

I’m developing an application that checks emails in a shared mailbox and moves them to different folders based on certain variables. So far, I’ve managed to do this using delegated access, logging in through a browser with my account. However, since I want to run it on a server, I need it to work without user authentication each time.

Everything works correctly with delegated access, and I can move emails in both my account and the shared mailbox. However, after switching to app authentication, I get the following error:

APIError Code: 403 Message: None Error: MainError(code='ErrorAccessDenied', message='Access is denied. Check credentials and try again.')

The Microsoft Entra admin granted full permissions to this application, which is why it works with delegated access. What could be causing this issue?

My code:

scopes = ['https://graph.microsoft.com/.default']
client = GraphServiceClient(credential, scopes=scopes)

query_params = MessagesRequestBuilder.MessagesRequestBuilderGetQueryParameters(
            select=['from', 'isRead', 'receivedDateTime', 'subject'],
            top=25,
            orderby=['receivedDateTime DESC']
        )
        
request_config = MessagesRequestBuilder.MessagesRequestBuilderGetRequestConfiguration(
            query_parameters=query_params
        )
        
        # 
messages = await client.users.by_user_id('******@majorel.com').messages.get(
            request_configuration=request_config
        )

Thank you.