Apart from Mail.app, the default client on Mac, Apple users pretty much only have Thunderbird as an alternate solution to check their mail from their desktop. However with Kiwi, Matt Roge is aiming to create a client that should be able to compete with those two.
The mail application is still in development, so you can’t download anything yet, but you can still sign up on the website to be notified of any announcement. Kiwi sounds like an ambitious project and we’re anxious to see if it can replace the efficient, but not excellent Apple Mail. Kiwi should integrate Address Book and Spotlight, be customizable (more than Thunderbird?) and should be able to handle up to 60, 000 messages. The developer also writes that “Kiwi is built off of MailCore, a Cocoa e-mail framework, and LibEtPan, a C based library for working with e-mail protocols.”
The screenshots look pretty basic, but you can still get a feel of the application. Seems to me like Kiwi will be aimed at those who receive lots of emails and want a simple tool to easily filter and organize them. For now the interface looks too similar to Mail.app to really know what will be put into it though. We’ll have to wait for future updates on the project to know more.