Thunderbird,
with the add-ons
Adblock Plus, CompactHeader and Lightning.
I've used it since (was it incorporated into Netscape 1995? I don't remember) then it morphed into Neoplanet, then into Mozilla where the integrated browser and email client were split into the separate Thunderbird and Firefox.
As an e-mail client it's excellent. One reason is, it isn't Bill Gates. Frankly, if you're a corporate person then dance with the devil and use MS Outlook instead. The Outlook environment has much higher capability (example: if you add the free MS Business Contact Manager then you can track projects, tasks within projects, etc.) But then, Thunderbird is free, while, alas!, Outlook funds Skynet. If you're a corporate person use Outlook. If you have independent livelihood (moi) then Thunderbird is great, (but vastly less capable than Outlook).
I've used both Outlook and Thunderbird a LOT, and I just LIKE Thunderbird. (I still use Outlook for contacts, because it's way better than Thunderbird re: Contacts)