Issue: 

Outlook shows warning that a calendar invitation cannot be sent on behalf of another mailbox

#550 5.7.1 RESOLVER.RST.NotAuthorized; not authorized ## – Calendar Invite undelivered when sent on behalf of another/secondary/shared mailbox

Resolution:

The mailbox from which the calendar invite has to be sent must be primary always. 

Option 1: Configure the shared /alternate/secondary mailbox as primary account in outlook profile

Option 2: Access Shared/secondary/alternate mailbox calendar from Outlook Web Access (OWA)/Webmail and send the invite.