Exchange 2010 users can open other mailboxes

Hello ;

I have migrated from Exchange 2003 to Exchange 2010 SP3 .

My issue is : some Exchange 2010 users are able to open some other mailboxes using Outlook or OWA !!

They don't have any permission on other users mailboxes , but they are able to open other user mailboxes !! I don't know why and how to fix this issue

I take an example mailbox  (user2) and checked all Mailbox permission  ... I cannot find that (user1) have any permission on user2 mailbox , however user1  still can open user2 mailbox and read his inbox emails !!!

note: user1 is not domain admin or exchange admin  ... just a normal domain user /member of domain users only !

I checked all permission in ADSIEDIT to make sure if user1 or any group which may have user1 member on it but I didn't found any un-normal permission assignment .

Please help

Thanks.