Also imagine your domain being joe-jobbed. You, as an innocent bystander, then get hammered by Verizon as they try to do a lookup on possibly millions of incoming mails.

Why on earth would Verizon need to do the lookup once per incoming email? If they need to verify that a given MX does indeed exist and is reachable and is running an SMTP server, then why not cache that info for some reasonable time period, say an hour, to avoid disrupting everyone else's Internet. Coupled with that caching, they could reasonably make a few tries over the space of 3 to 5 minutes before giving up on the incoming email by sending 450s. And if they are going to do something like this which imposes a requirement on other ISPs, i.e. your MX must point to a live SMTP server, and which impacts other ISP's mail operations, i.e. we will send you 450s, then why can't they *PUBLISH* what they are doing. NANOG seems an appropriate place for this. --Michael Dillon