Hi
To begin with you should disable your ex-administrators account, and remove from any high privilege Groups (Domain Admins, Exchange Admins, etc), next review these groups to see which accounts are members of these group, ensure you can account for all users who are in these groups, as well as users that are in nested groups.
Finally you will need to use Exchange System Manager (ESM) to view accounts that have been manually granted access to the Exchange Organization, to do this expand servers in ESM and right click properties on your server, open the security tab, and review user and groups that have access (repeat for each server), you will need to drill down further in ESM to your mailbox stores right clicking on properties and again reviewing the users and groups that have access.
If you want to do deeper I recommend looking into a script to enumerate permissions for each mailbox.
Regards
Ben Hoffman
Microsoft MVP - Windows Server System Exchange