If you install Exchange 2010 into a forest with multiple domains then you may come across this issue.
When you are trying to enumerate or move system mailboxes within your Exchange 2010 organisation, you will not typically get any results returned unless you use the -DomainController switch and specify a DC from the root domain as this is where Exchange creates most of the system mailboxes. Without the -DomainController switch, it will use a DC in your current domain, which won’t be able to see any accounts in its parent(s).
Get-MailboxDatabase | Get-Mailbox -DomainController <dcname.root.domain>
This will typically come up when trying to remove the first (i.e. default) mailbox database created when you installed your 2010 mailbox servers.