The designs we weren't interested in were done so because the M was a little difficult to make out. Also, we want to see things that are more clever than a lock. That's why we liked the first M so much. It also highlighted the fact that our messages are secure, not that we provide security.
I also like the arc of circles, but it doesn't seem to fit with the M. Maybe you could rearrange it or try to incorporate it in a different way. Keep your ideas coming! Everything has been great so far, but each time you submit we're better guided down our path.