I played PBEM Diplo before, but don't have the time to do it again. Sad, because I really liked it. But to do it well, you need to dedicate a lot of time discussing with your neigbouring states, and I just can't take another commitment.
Take a look at the site Ben pointed you too, NWOL is a fun game too. The mechanics are far more complicated, but if you just want to command an army from a major power it takes less time (since others do the diplomacy for you, basically).
I like both games, but only have time for NWOL right now.