What kind of program is Mozilla Thunderbird?

Mozilla Thunderbird is a free and open source desktop email client available for Windows, Mac, and Linux. It was originally developed by the Mozilla Foundation, the organization that created and maintains Mozilla Firefox, but they've abandoned it and it is not currently being developed by anyone, even though it is the most popular desktop email client.

Features:

• Manage multiple webmail, POP3, and IMAP email accounts from one location.
• Can help to block viruses, email scams, junk mail and spam.
• Allows for the installation and use of add-ons (some as the same as Mozilla Firefox).
• Can import address books, mail, filters, and other settings from other email applications (such as Outlook and Outlook Express).
• Full featured calendar add-on (separate download for Thunderbird 2 but will be bundled with Thunderbird 3).