Postbox is an email client that offers some interesting new features for this type of application.
It uses Mozilla technology to offer a useful and practical service. Its objective is to organize your email, making it easier for you to find messages, contacts, attached files, and images.
It has an advanced search feature that lets you use almost any criteria to find specific content.
The application is organized into tabs, which makes using it a lot easier and faster; and it also has a side bar that makes it easy to add content to new messages.
Postbox also integrates web services like Facebook, Twitter, and Friendfeed, so you can easily share your content between services. When you open Postbox for the first time, it asks you if you want to import your accounts from other mail clients, such as Thunderbird or others.