Thunderbird is a free, open source, cross-platform e-mail, news and instant messaging client. Thunderbird is a full featured email program with built-in junk mail controls, message filters, address book, news groups and more. It offers a clean and easy to use (customizable) interface. Thunderbird also offers security features such as S/MIME, digital signing, message encryption, support for certificates and security devices.

Other features include IMAP/POP compatibility, support for HTML mail, labels, quick search, return receipts, LDAP address completion, import tools and more.