Also imagine your domain being joe-jobbed. You, as an innocent
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.