I'll add that this question has come up from time to time. Occasionally members who receive such a treemail do not want it, and as replies continue to add up they continue to get annoyed with this tree mail popping up again and again on their homepage.
I have plans to allow recipients of group tree-mails to withdraw themselves from a treemail. I also want to let the sender have the power to "lock" the tree mail so that nobody can reply to it (except for the sender.)
