Here, we have 2 games that are candidates for a play-by-mail system:
www.vgaplanets.com
and
www.crypticcomet.com
Vga Planets is free (or was when I last looked), but Solium Infernum isn't. You'll have to ask about that one.
There is, however, a demo that you can try - downloadable from the website above.
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So far, for SI, we have:
Red_Avatar
TotalAnarchy
Saccade
El Quia
Fubb
Who else wants to join? Idk how many is max players
