Because this user is deleted from AD.
-Mailbox -Identity domain\user
The operation couldn't be performed because object 'domain\user' couldn't be found on 'dc.domain.yes'.
+ CategoryInfo : NotSpecified: (0:Int32) [Remove-Mailbox], ManagementObjectNotFoundEx
+ FullyQualifiedErrorId : 2F1637E6,Microsoft.Exchang
e.Manageme nt.Recipie ntTasks.Re moveMailbo x
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