The goal of the zMail project is to provide email integration within Eclipse. In addition to the traditional email client functionality, such as reading, writing, and storing messages, zMail aims to provide features specifically beneficial to OpenSource developers.
While it is true that there are several excellent platform-specific [http://www.microsoft.com/outlook/, http://www.novell.com/products/evolution/, ...] and several excellent cross-platform [http://www.mozilla.org/mailnews/, ...] mail clients, none of these tools integrate particularly well with the development process.
OpenSource development tends to involve creating (on the part of contributors) and applying and integrating (on the part of committers) patches to the source tree. The goal of zMail 1.0 is to explicitly support this functionality, as well as being a serviceable, cross platform, email client.