I would vote for #16. At first, it looks like a very simple design.Soylent logo is people!!!
(Sorry, couldn't resist).
I detest the little tails on many of these, representing speech balloons. That to me is clear iconography for chat or text message, not email or web groups. (Sorry Mark, I see that was the sample you gave them.)
But lose the tail and 16 makes for a nicely unique icon.
I like 10 too, for building an envelope out of network nodes. The only one that hints at web and email both. It would also make a unique Font Awesome character.
I also like how 15 and 17 build a G out of envelopes. Also a nicely unique icon, if a bit mechanical in appearance.
I suspect the best may be yet to come, given that there's almost three days left. Good design requires time to think, reflect, adjust, and think again.