So, I used this feature and discovered it works a little differently than a Dmail "blast" at Dave's Garden.
Whenever somebody responds to your multiple-recipient tree-mail, EVERYBODY sees the response. For some purposes, that might be a good thing. But this spring when I send a notice out to a hundred people to be sure Mid-Atlantic gardeners know about the spring plant swap... well, all those people do not need to see every "sorry I can't make it" or "do you realize I am 4 hours away" response... they don't even want to read the "I'll be there; can't wait" responses... those who are coming will connect up on the discussion thread.
Is there any way to do a multiple-recipient mailing so that replies are seen only by the original sender?