Transport servers store all incoming mail in a database file called mail.que until the transport server verifies that all of the next hops for that message have been completed. This database has an associated transaction log in which changes are first committed. If you are using an Exchange Server’s internal drives for storage in a high-volume environment in which one million or more messages are persisted, you should consider placing the database and the transaction log on separate disks for optimal performance.
With SANs, it might not be immediately apparent whether disks are physically separate. This is because the volumes you see are logical references to a portion of the storage subsystem. In this case, you might be able to use the Storage Manager For SANs console or a similar tool to help you select logical unit numbers (LUNs) that are on physically separate disks.
Transport servers have many different queues for messages. These queues are all stored in a single ESE database called mail.que. By default, this database is located in %ExchangeInstallPath%\TransportRoles\data\Queue. Thanks to shadow redundancy, the deletion of a message in the database is delayed until the transport server verifies that all of the next hops for that message have completed delivery. If any of the next hops fail before reporting back successful delivery, the message is resubmitted for delivery to that next hop.