The storage used by the Exchange guest machine for storage of Exchange data (for example, mailbox databases and transport queues) can be virtual storage of a fixed size (for example, fixed virtual hard disks (VHD or VHDX) in a Hyper-V environment), dynamic virtual storage when using VHDX files with Hyper-V, SCSI pass-through storage, or Internet SCSI (iSCSI) storage. Pass-through storage is storage that's configured at the host level and dedicated to one guest machine. All storage used by an Exchange guest machine for storage of Exchange data must be block-level storage because Exchange doesn't support the use of network attached storage (NAS) volumes, other than in the SMB 3.0 scenario outlined later in this topic. Also, NAS storage that's presented to the guest as block-level storage via the hypervisor isn't supported.