I looked further into this and I found what I think is happening. The system tries to prevent duplicate tree mails from being sent, for example when someone double-clicks the "Send" button instead of single clicks. It does that by checking several factors, but most notably is the subject line.
Until this morning, you could not send a tree mail to different people using the same subject line inside of a 15 minute window. So I suspect that's what happened: you were trying to send mail to several people using the same subject line.
Just now I shortened that window down to 90 seconds. So you can send a tree mail and 90 seconds later you can send the same message again to someone else.
There's probably an even better way to do this and I'll work toward that, but at least for now this should fix most problems with mails not getting sent.
