Paul Tyma, creator of the great service Mailinator wrote a post about the architecture of the system. The reason I point to it is because of the simplicity/beauty of the system. Here is a short couple quotes:

At this point, Mailinator averages approximately 2.5million emails per day. I have seen hourly spikes that would result in about 5million in a day. In addition, the system also services several thousand web users and several thousand RSS users per day.

In the world of email services, this probably isn't all that much. The most interesting part to me is that the complete set of hardware that mailinator uses is one little server. Just one. A very modest machine with an AMD 2Ghz Athlon processor, 1G of ram (although it really doesn't need that much), and a boring (IDE , low-performance) 80G hard drive. And honestly, its really not very busy at all. I've read the blogs of some copycat services of Mailinator where their owners were upgrading their servers to some big iron. This was really the impetus for me writing down this document - to share a different point of view.

Reminds me of how Brent and I typically approach similar ideas. Take SimpleFilter and Blogchat for instance. Without going into details, they both were designed to run and operate as very simple services on extremely modest hardware but also with the ability to scale well.