I am (mercifully) not a hardcore email-systems weenie

but I find it hard to believe that this imaginary company is really going to be able to break even, never mind make a profit, charging a single penny per message. It adds up, sure, but the other side of the equation is that you've got to pay for a lot of hardware to handle the traffic, the resources necessary not only to validate a message but also to either sign the ones that are good or store the ones that aren't. (This is the part where the lawyers start thinking about cases where your magic SMTP company manages to flag an important correspondence as bad whereupon it gets sucked into the vortex. Heaven help us when we teach computers to think like lawyers...) Even if you only store errant messages for, say, no more than a week, that's a huge amount of disk space. Mulitplied by two, three or four depending on your backup strategy. On top of all of that, your bandwidth charges -- no matter how sweet a deal you cut with a backbone operator -- are going to make someone else very very rich. And were that already not enough, you will have to pay some very real salaries to the sysadmins that have to watch the whole thing because it will quickly become every one's favourite target for hacking or just otherwise poking. It also becomes a single (or more single) point of failure, a probably uninsurable liability, a thorn in the side of every accounting department for any business that does stuff online (you don't seriously expect them to absorb that cost or the cost of massaging their email systems, do you?) and an opportunity for someone to engage in the kinds of data-mining that no one wants. It would already be a tough sell to consumers at one cent a message. It would be dead in the water at anything more.