  Web services offer an opportunity to redesign a variety of older systems to
exploit the advantages of a flexible, extensible, secure set of standards. In
this work we revisit WSEmail, a system proposed over ten years ago to improve
email by redesigning it as a family of web services. WSEmail offers an
alternative vision of how instant messaging and email services could have
evolved, offering security, extensibility, and openness in a distributed
environment instead of the hardened walled gardens that today's rich messaging
systems have become. WSEmail's architecture, especially its automatic plug-in
download feature allows for rich extensions without changing the base protocol
or libraries. We demonstrate WSEmail's flexibility using three business use
cases: secure channel instant messaging, business workflows with routed forms,
and on-demand attachments. Since increased flexibility often mitigates against
security and performance, we designed WSEmail with security in mind and
formally proved the security of one of its core protocols (on-demand
attachments) using the TulaFale and ProVerif automated proof tools. We provide
performance measurements for WSEmail functions in a prototype we implemented
using .NET. Our experiments show a latency of about a quarter of a second per
transaction under load.
