Exchange via EMC:

This will search all of the mailboxes to determine which a user has FullAccess permissions:

[PS] > Get-Mailbox | Get-MailboxPermission | Where { ($_.AccessRights -eq “FullAccess”) -and ($_.User -like “DOMAIN\USER”)} | FormatList

RunspaceId : 12345678-1234-5678-9012-123456789012
AccessRights : {FullAccess}
Deny : False
InheritanceType : All
Identity : DOMAIN.LOCAL/Users/Firstname Lastname
IsInherited : False
IsValid : True

It can take a while, but it will give you the results.