XM Mail Server is designed around a highly sophisticated pipelined asynchronous architecture. Typical MTA's use a separate virtual processor, or thread for each new connection. This causes the CPU to rapidly switch between threads wasting time if that thread is not used.
With XMS ARC technology, work is broken down into smaller modules and delivered through a pipeline to a very small number of threads. This eliminates the typical switching overhead, and allows the CPU to keep more useful information in it's cache, making memory and disk access much more efficient. This allows XMS to tackle the "C10K" problem that most MTAs are plagued with. In other words XMS can efficiently support over 10,000 concurrent connections.
The more concurrent connections, the greater the number of recipient servers you can communicate at one time. This has an exponential impact on server resources and overall speed.