MailboxNotEnabledForRESTAPI — Mailbox not enabled for REST API

The user’s mailbox isn't available via the Graph API (perhaps it's an on-premises or inactive mailbox). Riposte cannot access such mailboxes because Microsoft Graph requires an Exchange Online mailbox. The only solution is to use a mailbox that is in Exchange Online or re-enable the mailbox. You should alert the user that their account isn't supported for this integration. In your application, you might remove or pause this account (via /accounts/{accountId}/pause) since it cannot be used.