When I installed sendmail 8.12.8, I did the following (among other things

)
added strings
FEATURE (`access_db')
dnl this sets DNS-based ACL
FEATURE (`relay_hosts_only')
dnl treating records in ACL as the names of hosts, not domains
to the file <sendmail_pkg>/cf/cf/sendmail.mc and built it
cd <sendmail_pkg>/cf/cf/sendmail.mc
./Build sendmail.cf
./Build submit.cf
After the binaries and *cf were installed, I created /etc/mail/access with records like
localhost RELAY
localhost.localdomain RELAY
# consult your /etc/hosts for exact FQDN of your localhost
127.0.0.1 RELAY
my_left_hand_side_neighbour.myoffice.my.net RELAY
my_right_hand_side_neighbour.myoffice.my.net RELAY
# any hosts not listed here are denyed by default
and compiled it with
cd /etc/mail
makemap hash access.db < access
Note, that this may be circumvented by dedicated software that can answer forged hostnames!!!