I use claws-mail. Not exactly fancy but it's fast, stable, and can handle several GB of data in inboxes even in slow machines without a hiccup. My second optio

I use claws-mail. Not exactly fancy but it's fast, stable, and can handle several GB of data in inboxes even in slow machines without a hiccup. My second option, beacuse of the features it offers, would be Kmail/Kontact.