Mozilla Thunderbird is a free, open source, extensible, cross-platform email, news, and chat client developed by the Mozilla Foundation.
Thunderbird is an email, newsgroup, news feed, and chat (XMPP, IRC, Twitter) client. It supports POP and IMAP methods of connecting to email servers. It also supports LDAP (directory) address completion. The built-in RSS/Atom reader can also be used as a simple news aggregator. Thunderbird supports the S/MIME standard, extensions such as Enigmail add support for the OpenPGP (security)standard. The vanilla version is not a personal information manager, although the Mozilla Lightning extension adds PIM functionality. Additional features, if needed, are often available via other extensions.