Thunderbird is a free, open source, cross-platform e-mail, news and instant messaging client. The project strategy is modeled after Mozilla Firefox. It supports POP (storing mail locally on your hard drive so that it can be accessed without an internet connection), and IMAP. It features excellent mail filter capabilities and management. Thunderbird has strong support for using multiple accounts and identities, including automated signature features.
- It has all the calendaring features and email management features of outlook but is open source
- For me it is the only alternative to Outlook, if you want power of Outlook and open source there is just no other app.
- Provides most of those features Outlook offers to users for professional work. The missing features are very specific, and with the additional security, still a nice trade off.