(susan, this is in a spam related thread but i'm adding offtopic remarks which i think are actually in-charter for nanog. --pv)
Verizon does SMTP callbacks, connecting back to the MX of the envelope sender and trying to verify that the user exists
while something like RMX or MAILFROM would probably be a more robust alternative, verizon's actions are not irrational on a purely cost:benefit basis when the costs and benefits being measured are only their own. however, cost and benefit are not isolatable in that way, and folks who try to isolate them end up causing others to pile workaround on top of workaround until the whole system is just gum and mud. if verizon wanted to jointly sponsor a clearinghouse of email senders who had passed the callback test, with appropriate caching and error analysis and robust global mirroring, i'm sure that there would be other isp's and large e-mail carriers who would want to help, and i'm sure that authors of mail software, both opensource and not, would want to offer the feature of checking such a "ephemeral sender whitelist" (ESW?) but as long as verizon acts alone, they're just hurting themselves, and the overall system. consider what would happen if everybody did callbacks; first, what would happen to the load on the world's nonabusing mail servers, and then, what would the spammers do in response if this was effective? -- Paul Vixie