I have a friend who uses pobox.com for spam filtering and mail forwarding. I have a lot of respect for pobox; they're one of the oldest commercial services on the net (1995) and they didn't do anything stupid during the Internet boom. Alas, my friend stopped getting email from me.

X-Pobox-Antispam: Looks like broadband returned DENY:
adsl-63-194-75-26.dsl.snfc21.pacbell.net looks like a consumer broadband machine (contains 63.194.75.26)

And as their online docs say:

Looks for signs that a message came from a DSL or cable modem user, directly. Most DSL or cable modem users should send mail through their ISPs' outgoing mail servers. This is by far our most effective condition, catching upwards of 20%% of all the mail caught by Spam Protection. To make sure it is active on your account, go set up Spam Protection, and set "I correspond with people who run their own MXes on broadband lines." to "NO".

However, if you know Unix/Linux users who run their own mail servers on their broadband lines, you probably do not want to activate this blacklist. You should set that question to "Yes".

Alas, my friend's account was defaulted to "No" without notification a few weeks back and he stopped getting my mail. No bounces, just silently dropped.

This filter is an example of the end of SMTP. Internet email was designed so that any one computer could send mail to any other computer. Compared to centralized mail systems (think Compuserve) this is a radical design. Alas, in the era of spam zombie machines it's not working any more. Pretty soon we're all going to have to route our email through ISP-approved mail servers.

Pobox seems to be doing a simple whitelist of their own. The real switch will be wide deployment of a system like DomainKeys, where ISPs use DNS and digital signatures to indicate "this IP address is a legitimate mail sender". Every other IP address (like mine) will then be suspect.