Most email servers end up being I/O bound. On email gateways, the primary source of I/O contention involves the mail queue.
Email I/O is (1) write-intensive, (2) synchronous, and (3) characterized by small and random accesses. Using a high-performance filesystem that gracefully handles directories with many entries and efficiently but safely handles synchronous writes can be an enormous win.
Storage space rarely emerges as an issue in an email queue unless something goes wrong. A disk's suitability for this purpose should be determined by how many operations per second it can perform.
For email performance, we're primarily interested in increasing the amount of email that can be moved on and off a machine per unit time. While the latency of the transaction represents a secondary concern, reducing latency remains a good idea, as long-lived connections consume resources.
