Any user can “Send on Behalf” with Exchange

Today I was instructed to help our company Systems Administrator with a problem of “Send on Behalf”. It appears that some users could send mails on behalf of other people, but those users didn’t have the rights set within the Exchange Admin Center, as seen in the screenshot below. So, how is it possible that those users could still send mails on behalf of others in our organization? It looks like the problem resided in our migration path from Exchange 2003 to Exchange 2007 and a few years later to Exchange 2010 and we are in…

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