A> SPF-like proposals also scale well
Something that requires pre-registration of all of the places
I may send
mail from does not scale all that well. Imagine having to preregister
every telephone you might call from.
At least that's what my phone company calls them - effectively a credit card
for making local and long distance phone calls without depositing money.
It's my "registration." But that's more analogous to SMTP AUTH, or SMTP AUTH
over a different port.
Now this was covered in the LMAP discussion draft, too. There are many ways
for authorized users to authenticate themselves against a relay server for
sending mail. Just like calling cards let me place long distance calls at my
phone company's rates from a phone not serviced by my phone company. And as
explained in said draft, they don't have to scale with the Internet, just
with the number of users at a given site.
The die-hards who just have to be their own MTA can use dynamic DNS to
"pre-register" and "de-register" their locations seamlessly. I believe these
to be in the extreme minority but they're not excluded. This is an
implementation problem, not a design problem for us. It's an easy problem to
solve too, at least for the IP-based designs like DMP and FSV.
 If folks are going to say "spf-like," "rmx-like," I am going to say it.
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GOD BLESS AMER, er, THE INTERNET. <http://vmyths.com/rant.cfm?id=401&page=4>