Sendmail: How does one blacklist annoying spammers?

[Genes MailLists]

DKIM signed emails:

  Not directly anti-spam but very related.

 Major outbound MX mail is generally signed by DKIM (or domainkeys).
While I am not yet making rejection decisions on sigatures that fail - I
do have my mail client color them green/red when a DKIM/DK sig is good/bad.

  I also ensure my own domains sign all outbound mail with both DKIM and
DK (for yahoo!).

  I use thunderbird and while its easy enough to set up I do with the
mail client folks would do this by default.

  SO it is very easy for me to see in my mail client when I have a good
signed email from gmail or a fake one.

  gene