I am mildly confused… the system emails are basically emails from you to you so they don’t have any of the CANSPAM stuff like an unsubscribe button. Something else may be preventing the emails but it isn’t any link you would have clicked.
Outlook 2013 also gives you the option to unsubscribe if it detects an email my be of a commercial nature. Check your spam folder &/or configure your client to whitelist the pbx’s email address.