I am trying to open another mailbox from Outlook/Access a different email account using the Graph API. Within the exchange admin centre, I have enabled Full Access to the mailbox I am trying to access. However when I try open the account the account from Outlook using 'Open Another mailbox' I get this error:

UTC Date: 2022-01-20T04:32:42.882Z
BootResult: accessDenied
Client Version: 20220110007.08
err: Microsoft.Exchange.Data.StoreObjects.AccessDeniedException
esrc: StartupData
et: ServerError
estack: Microsoft.Mapi.MapiExceptionNoAccess
st: 500
ehk: X-OWA-Error
efe: SY6PR01CA0002
ebe: SY4PR01MB6948
ewsver: 15.20.4909.8
emsg: ErrorPermissionDenied

I've also tried doing this through the Graph Explorer using the endpoint (https://graph.microsoft.com/v1.0/users/userId/messages)But I get an error saying the folder is unavailable.

{
    "error": {
        "code": "ErrorFolderNotFound",
        "message": "The specified folder could not be found in the store.",
        "innerError": {
            "date": "2022-01-20T04:41:59",
            "request-id": "4f835246-029b-47af-86d5-15f5dbb52bde",
            "client-request-id": "20ae4819-d85e-dd92-fe6b-14fda438e947"
        }
    }
}

Am I missing some permission set or something? The user I am using I believe is an admin.