Matthew Elvey raises two points that I summarize as follows:
1. If you don't know what a domain's outward-facing MTAs are,
it's really hard to build a SenderID record. (And he estimates
that half of his customers are in this boat.)
True.
2. If instead of SenderID, we'd merely authenticated the HELO name,
and built up reputation systems that say whether particular
HELO names behave reasonably, these people's lives would be easier.
It's true, but it's a whole lot weaker form of authentication. I don't
care whether a mail server is who he says he is, I care whether he's
authorized to send the message he's trying to send.
-- Jim Lyon