J.R. von Behren, S. Czerwinski, A. D. Joseph, E. A. Brewer, and J. Kubiatowicz.  NinjaMail: The Design of a High-Performance Clustered, Distributed E-Mail System In the
Proceedings of "International Workshops on Parallel Processing 2000".  August 21-24, 2000, Toronto, Canada.  (Edited by P. Sadayappan).  pp 151-158
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Abstract:

In today's Internet era, electronic mail is replacing telephony and postal mail as the primary means of communication for hundreds of millions of individuals. Free email services, such as Microsofts Hotmail and Yahoos Ya-hoo!  Mail, each have tens of millions of subscribers. However, these and other current email systems unfortunately are not capable of handling the scale of Internet email use, while still providing reliable, high performance and feature-rich services to users. This limitation is the result both of suboptimal use of cluster computing resources, and of highly variable performance of wide-area connections over the Internet. This paper presents NinjaMail, a novel geographically distributed, cluster-based email system
built on top of UC Berkeleys Ninja cluster architecture and OceanStore wide-area data storage architecture. NinjaMail is unique in that it uses a collection of clusters distributed through the wide-area to provide users with highly available, scalable and feature-rich services via a wide variety of access methods.