Mulberry is an email and calendar client for Mac OS X, Windows and Linux .
Mulberry started off life as a software project that was really meant to help the author learn more about the internet and internet protocols used for email. However, it became much more than that and garnered support from a small (in internet terms) group of users and institutions many of whom relied on the product as their primary email tool.
Whilst it started as only an IMAP client and only on Mac OS, it has grown to cover not only other email protocols, but also calendaring and scheduling and is available on Mac OS X, Windows and Linux systems.
Adherence to standards is a key belief on the part of the developers, and active participation in the Internet Engineering Task Force and the standards process is an important aspect.
Ultimately the failure of the original company (for a whole host of reasons) was really a reflection of the fact that Mulberry had failed to grow a market share that could sustain the company in a market where free clients (of varying quality) are readily available.
Now Mulberry itself is free.
Download: Mulberry 4.0.9a1