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Thanks for the feedback.
Hi Network Admins,
I understand that you have set up single sign-on, and would like to access other mailboxes. Given this situation, to achieve this goal, I suggest you try the following workaround:
1. Connect Windows PowerShell to the Exchange Online service, refer to http://help.outlook.com/en-us/140/cc952755.aspx.
2. Run the following command to grant Full Access permission:
Add-MailboxPermission "email@example.com" –User useraccount -AccessRights FullAccess
3. Use the Open Other Mailbox feature to access the mailbox firstname.lastname@example.org. You can refer to http://help.outlook.com/en-us/140/bb899681.aspx.
How are the things going? Do you need any further assistance on this issue?