Thunderbird

Mozilla Thunderbird is a free, open source, cross-platform email, news, and chat desktop client developed by the Mozilla Foundation.

• As of version 15, Thunderbird supports instant messaging and chat using IRC, Facebook, XMPP, Twitter, and Google Talk.
• As of version 38, the Lightning calendar addon is bundled, with an opt-out dialog

Thunderbird is an optional part of the Tiki Suite. If you don't need a desktop client, but only a webmail, just use Kolab or Zarafa.

See also: SeaMonkey