Obrigado a todos - isso foi muito próximo - mas quando o executei em alguns servidores diferentes de 2010, recebi um erro:
Pipeline not executed because a pipeline is already executing. Pipelines cannot be executed concurrently.
+ CategoryInfo : OperationStopped: (Microsoft.Power...tHelperRunspace:ExecutionCmdletHelperRunspace) [],
PSInvalidOperationException
+ FullyQualifiedErrorId : RemotePipelineExecutionFailed
Pipeline not executed because a pipeline is already executing. Pipelines cannot be executed concurrently.
+ CategoryInfo : OperationStopped: (Microsoft.Power...tHelperRunspace:ExecutionCmdletHelperRunspace) [],
PSInvalidOperationException
+ FullyQualifiedErrorId : RemotePipelineExecutionFailed
que me levou a link que sugeri usar uma variável - então meu resultado final foi:
$mailbox = Get-Mailbox
$mailbox | % { Get-MailboxPermission $_ | FL | Out-file "$_.txt" }
que funcionou perfeitamente! Obrigado novamente!