Sizing the MTA Queue disks are an important step for improving MTA performance. The MTA's performance is directly tied to disk I/O first above any other system resource. This means that you should plan on disk volume that consists of multiple disk spindles, which are concatenated and stripped by using a disk RAID system.
End users are quickly affected by the MTA performance. As users press the SEND button on their email client, the MTA will not fully accept receipt of the message until the message has been committed to the Message Queue. Therefore, improved performance on the Message Queue results in better response times for the end-user experience.