For the previous post in this series, go here first.
Microsoft is indeed the sole arbiter of who gets to send email to hotmail.com. As is Curt the sole arbiter of who gets to send email to monash.com, and I to richi.co.uk: My server; my rules.
If I chose to reject email from, say, Turkey, because I'm getting a bunch of spam from there, that's entirely my prerogative. I'm not forcing anyone else to reject email from their friends in Turkey.
As far as call-to-action filtering is concerned, I think Brightmail proved that a signature-based approach can be useful. Their approach includes CTA, but is a more general signature filter. It can be labor-intensive however, and suffers from a "zero hour" problem.
It's notable that the two major hosted anti-spam companies (MessageLabs and MX Logic) who use Brightmail use it in conjunction with their existing techniques—not as a replacement. In addition, Symantec (Brightmail's new masters) use the TurnTide IP-level throttling technology in front of Brightmail in their appliances. From accounts, this means that a substantial chunk of the inbound spam never gets as far as the Brightmail filters. This makes perfect sense for the reasons I outlined here.
Ultimately, if Microsoft screws this up and drops too much mail, the market will decide that Hotmail's had it's day. For all its faults, Microsoft's not that stupid. My "faith," if that's the right word, is not about "infallible wisdom and technical perfection," it's about revenues from advertising, partnerships, and the many other ways that MS derives a revenue stream from MSN, direct and indirect. Hotmail ain't a charity any more than Gmail, Monash Information Services, or Richi Jennings Associates are.
Tags: spam, Hotmail, SPF, SenderID, Sender ID.