Blocking SPAM at the source?

From inet-access

One way to fix this would be for the sites that own large dialin pools to
firewall their dialins so that they can only do SMTP to their own servers.

Is anyone doing this like IBM Advantis, InternetMCI, AT&T Worldnet, etc?

If not, why not?

Michael Dillon - Internet & ISP Consulting
Memra Software Inc. - Fax: +1-250-546-3049 - E-mail: