Sendmail: How does one blacklist annoying spammers?

[Bruno Wolff III]

On Mon, Jun 28, 2010 at 17:34:38 +0800,
  Ed Greshko <Ed.Greshko at greshko.com> wrote:
> 
> AFAIK, SPF records are queried by the receiving based on the domainname
> in the "From" address.  So, if a spammer is pretending to be sending
> from user at hotmail.com the receiving end will make a DNS request for the
> TXT records of hotmail.com to determine its SPF records and how to treat
> the email after the analysis.

SPF refers to the envelope sender address. That's why it causes problems
with forwarding. Normally the address in the from header matches the
envelope sender address, but that isn't always the case (mailing lists
being a big exception).