Have you tried performing the IEMSTEST for the two mailboxes? If not, it would be recommended you do so.
Also, please ensure these mailboxes are not a part of Protected Group, which are:
* Administrators
* Account Operators
* Backup Operators
* Cert Publishers
* Domain Administrators
* Enterprise Administrators
* Print Operators
* Schema Administrators
* Server Operators
The following user objects also are protected:
* Administrator
* Krbtgt
If so, refer here:
View Document
Hope it helps.
Cheers