Geary is a mail client for Linux with a similar interface to conversation view. It's still fairly young, so not entirely polished, but it is usable. Edit: usab

Geary is a mail client for Linux with a similar interface to conversation view. It's still fairly young, so not entirely polished, but it is usable. Edit: usable, but seemingly with no support for encryption/signing, among various other things. Maybe it needs a bit more time to be ready.