From the looks of the existing XenForo_Mail class, it looks like a queue system is already planned. One thing I would suggest would be to allow an optional priority of sorts you can set when queuing something. For example say I sent an email to 500,000 users (you could of course argue that maybe you shouldn't be sending that many from the AdminCP, but just for sake of argument...) So now I have 500,000 emails inserted into the queue and they may take quite awhile to send (hours)... you wouldn't really want the higher priority stuff in the queue to have to wait (things like registration emails) for the bulk stuff to finish. Two different queue senders... a "normal" and a "bulk" would be ideal in my opinion. Then you don't have the queue senders affecting each other.