The following script provides an output of any login that has access to a mailbox not owned by that login.
The output should resemble the following screenshot:
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USE ArchiveManger
SELECT l.LoginID,
l.DisplayName AS OWNER,
mb.MailBoxID,
mb.Name AS MailboxName
FROM LoginMailBox AS lm
JOIN mailbox AS mb ON lm.MailBoxID = mb.MailBoxID
JOIN login AS l ON lm.LoginID = l.LoginID
WHERE NOT exists
( SELECT top 1 1
FROM mailbox AS m
WHERE m.MailBoxID = lm.MailBoxID
AND m.OwnerLoginID = lm.LoginID )
ORDER BY OWNER
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