Traveller is the prototype Science Fiction game, first created back in the late seventies after Dungeons & Dragons. As such, the game has a very old school feel to it, with six stats, hit points and all the trappings. When it first came out it had the unique distinction of being the first game in which your character could actually die during character creation, as the system created your history for you as you went through the process.
With a loyal following and over forty years of input, Traveller has seen many, many changes and editions through the years, complete with a GURPS version, and who knows, maybe even a D20 conversion. The history of the universe is long, convoluted and detailed, allowing for nearly endless possibility for stories. Traders, politicians, spies and soldiers can all find an era to be happy in, sometimes all at once.
If you like old school games, Traveller is for you, and works fine in Play By Post.