A very little talked about desktop mail client. It requires JAVA to run (boo!) and uses a lot of resources. If you have 4GB+ RAM on your system, give it a shot. It supports IMAP and POP protocols apart from Zimbra Collaboration server for which the mail client was actually built.
Even though it is slow to start, I personally find the mail client to be stable and works well . Like Thunderbird, it allows add-ons called Zimlets, there aren’t many but a few nifty zimlets are available. It is feature rich, supports one of the best threaded views among mail clients, can autoconfigure GMail and Yahoo! e-mail accounts and also supports Microsoft Exchange IMAP protocols. Zimbra desktop client is cross platform and works in Linux, Mac and Windows. You can download Zimbra Desktop Client from this link. I will place Zimbra Desktop client in my list ahead of KMail because of its features and stability.