Mozilla Thunderbird is the most popular open source email / news-feed client. It is free and available for all operating systems such as Windows, Mac OS X (Leopard, Lion..) and Linux based OS such as Ubuntu 10.10, 11.04, 11.10.
Mozilla has officially released Thunderbird 24.4.0, an email and RSS client, for all the available platforms, including Linux. A new version has been released in the Thunderbird 24.x branch, but there is nothing to be excited about, a fact that has unfortunately become the norm for new Thunderbird releases.
Mozilla has officially released Thunderbird 17.0.8, an email and RSS client, but the company hasn't said yet what the changes for this version are. Mozilla Thunderbird 17.0.8 is not a major release and this can be easily seen from the complete lack of information about it.