Windows: Mozilla's Thunderbird is a solid email client, but it still doesn't support 64-bit processors. FossaMail is a free clone optimised for 64-bit systems, all while supporting the same extensions.
FossaMail is made by the developers behind Pale Moon, an optimised version of Firefox. It creates a separate profile folder on your computer, thus letting you use it separately from your Thunderbird too. Setup is smooth and easy, and you'll soon have a functional email client that looks and feels just like Thunderbird, only faster. It also seems to be lighter on resources; where Thunderbird hovered around 220MB for us, FossaMail was steady at 180MB.
The makers of FossaMail have also created a 64-bit version of the calendar/organiser add-on Lightning. You can also get popular Thunderbird plugins by downloading the XPI file from Mozilla Add-Ons and manually installing it in FossaMail.