The other day I had to look at performing a mailbox export in an exchange environment that used child domains, the exchange servers were in the top level domain and users in a child domain.

The usual get-mailbox command would not work form me as it was only searching the TLD.

After a bit of searching I found the –IgnoreDefaultScope option however this still refused to work for me, I kept getting:

So I found the Set-ADServerSettings command:

Set-ADServerSettings –ViewEntireForest $True

After that commands worked a treat!

Hope it helps