Mozilla is the open source developer of several powerful programs, including the well-known Firefox browser. The same operation produces Thunderbird, a powerful email client and Sunbird, a standalone calendar and scheduling program.
People looking for Outlook alternatives naturally turn to Thunderbird because of their familiarity with Firefox. The good news is that Thunderbird typically uses fewer system resources than Outlook and offers what some people believe is a better user interface. The program also offers unique features such as the capability to display RSS feeds and news groups while allowing users the ability to customize the applications with personas and themes.
Thunderbird supports POP, SMTP and IMAP internet mail and comes equipped with wizards to help with configurations. When paired with the Mozilla Lightning add-on, Thunderbird can replicate many of the calendar and scheduling features of Outlook. Unfortunately, Thunderbird does not natively function as an Exchange client, but access can usually be configured using IMAP. Still, that could represent a limitation for many users. An add-on available for Thunderbird is available that allows that application to access Exchange calendars.
Another strong feature of Mozilla Thunderbird is that it is available on Window, Mac OS, and Linux, so environments with mixed platforms can interact with a common user interface.
Thunderbird is available as a free application.