Thunderbird is a free cross platform e-mail application that is easy to use and customize.
Ranked in these QuestionsQuestion Ranking
Works as expected, version after version.
Pro Large assortment of plugins
App design has not changed significantly since its beginning, making its learning curve negligent.
Pro Good filtering system
Flexible filtering system with the ability to set flags and read/unread as well as sort/assign to new mail directories. Numerous plugins to assist in the filtering, as well.
Con Development slowed to only focus on stability and security
Thunderbird has slowed down it's development of features to only focus resources on security and stability.