While I was getting a haircut today, I thought about the scalability problem of solutions like TEN and SMTP4All.

Digitally signing each message will reduce mail throughput significantly.  Some of the throughput loss will be offset by removal of spam which will reduce message traffic by as much as 90%.  Scalability problem remains still because mail traffic is not constant.

Add to this the cost of bi-directional authentication between sender-SMTP and receiver SMTP.  If messages per session is high (i.e. mail traffic between AOL and MSN), cost will be minimal.  But I suspect the average number of messages per session is pretty low, meaning near one message per session.