This seems to have been a FAQ with the most common answer being "that's
a stupid thing to do."

At any rate, I think it's worthwhile for my domain, so this is what I
added to my sendmail.cf:

SLocal_check_mail
R$* $: $1 $| $&{client_addr}
R< $* @ $* highlandsun.com > $| 66.221.212.169 $@ <OK>
R< $* @ $* highlandsun.com > $| $* $#error $@ 5.5.4 $: "553 You
agree to pay me US$1000.00 for each use of my domain name, payable
monthly. Repeated use constitutes your acceptance of these terms."
R< $* @ localhost $* > $| 127.0.0.1 $@ <OK>
R< $* @ localhost $* > $| $* $#error $@ 5.5.4 $: "553 Real domain
name required for sender address"

Since all of my outbound email originates on this machine, anything
coming from elsewhere is clearly an abuse.