Zimbra is a collaboration suite centered around email server. It has a rather a long and tumultuous history. Founded in 2005 it was sold to Yahoo in 2007, which sold it to VMWare in 2010, which sold it to Telligent in 2013, which then decided to switch its name Telligent to Zimbra. If you are looking for a free Zimbra alternative, available both in cloud and as self-hosted software, with access to source code, take a look at Bitrix24.
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.