Better than Thunderbird?

http://www.postbox-inc.com/

Postbox is based on Thunderbird. Thunderbird lacks PIM (personal
information manager) capabilities. This program ads that to
Thunderbird. If you are a basic user of email, Thunderbird is lean and
it's all you need. However if you live and die by your email, you need
PIM capabilities. Outlook does a good job at this, but's it's not
free, and some people just refuse to use anything from Microsoft.