Fading Suns is a dark fantasy set in the far flung future. While replete with Science Fiction trappings, the setting is deeply mystical, and includes both magic and faith based mysticism. It very much mirrors medieval Europe during the Dark Ages, with a little bit of Dune thrown in to keep it interesting. There are guilds, noble Houses, and the Church of the Pancreator, with several Orders within it. High tech exists, but most people don't really understand it, and only a rare few can repair it. There are aliens, many of them subjugated to the Church. The others are dedicated enemies.
The game itself is reminiscent of Dungeons & Dragons, although the magic system has several differences. It isn't class/level based, which for my money is a good thing. My play experience is limited, so I can't really speak to the game flow. The world, however, is well developed, with a dark tone and deep politics. There are lots of stories to tell, and several directions to go.
It would work all right as a play by post.