Install sendmail 8.8.2.  Generate a the file yourdomain.mc and append
the foo.mc stuff to the bottom.  Make the *.cf file and install it as
sendmail.cf.  Put the spamlist file and the mxlist file in place.
Restart sendmail.

===

This is what I use for my foo.mc macro file under sendmail-8.8.2.
Implements both sender domain blocking and 3rd party relay blocking.

Put spamsites in /etc/spamlist.  eg:

        CyberSpam.com

Put your MX's in /etc/mxlist (or leave empty if you don't MX's for
anyone).  eg:

        mymxclients.com

===== foo.mc ======

LOCAL_CONFIG
F{SpamList} /etc/spamlist
F{MyMxs} /etc/mxlist

LOCAL_RULESETS
Scheck_compat
R<$+> $| $+                     $1 $| $2
R$+ $| <$+>                     $1 $| $2
R$+ ! $+ ! $+ $| $+             $3@$2 $| $4
R$* @ $* $={SpamList} $| $*     $# error $: 552 SpamFilter: email from junkmaile
r's domains not accepted.
R$* @ $* $={MyMxs} $| $*        $@ valid from
R$* $| $* @ $* $={MyMxs}        $@ valid to
R$- $| $*                       $@ unqual local from
R$* $| $-                       $@ unqual local to
R$* $| $*                       $# error $: 553 ForwardFilter: We don't forward 
email from $1 to $2.

=== end of foo.mc ===