>> Could this be a good application of the inetd functionality that mrg
>> recently added?  i.e. inetd checks the IP address (or domain) of the
>> machine trying to connect to the SMTP port, and refuses the connection
>> if it matches a known spam engine.

>You would have to make inetd launch sendmail, then.  In the default
>configuration, sendmail is running all the time and receiving the
>connections by itself.  Starting sendmail from inetd could cause a great
>performance loss on busy mail servers...

  Not anymore (as of 8.8.0): sendmail can be configured to use
libwrap--just add -DTCPWRAPPERS to CFLAGS and -lwrap to LDFLAGS.

--Jim