There already is at least one open source email client for Mac OS X that supports IMAP well, and that is Mozilla Thunderbird.    That’s not to say that it is the best, and that there isn’t room for another (or several), because there is plenty of room, and I encourage it.

If you want a lot more buy-in from developers and users alike, make it multiplatform; write it in Qt (http://qt.nokia.com).  Qt does a very good job of making the Mac version of an application Mac-like, while the Windows version, is Windows-like.    The Linux version; well, is mostly Windows-like too

It is very easy to write a Qt application that is every bit as “Mac-like” on the Mac as one written in X-Code.    The big difference, is you then have a product with a MUCH bigger audience.

Many home Mac users are forced to use other operating systems at work (Windows the the main OS where I work, but I am fortunate to be a Linux developer, and spend most of my time in that environment at work).    I really like the Mac Mail client, but alas, can’t use it at work.  I use it on my Macs, and Thunderbird on Linux and Windows at work.

If someone wrote a killer email client that ran on all three; I’d use it everywhere.

@Jay,

What are some examples of existing “Mac-like” QT apps?

@Jeff Gamet

I think one thing that would help considerably would be an official Apple-designed plug-in architecture for Mail. Currently, the plug-ins for Mail are hacked in using Objective-C method spoofing thanks to the work done by Stéphane Corthésy of Sente in Switzerland who spent a lot of time reverse engineering Mail. He has repeatedly asked Apple over the years to officially support a plug-in system.