I had this same problem and how I got it to work was by adding this to /etc/hosts.allow
sendmail:127.0.0.1,172.28.108.71
you will need to sub in your own IP for the 172
and having sendmail listen on both the loopback and public interface
in /etc/mail/sendmail.cf.
O DaemonPortOptions=Port=smtp,Addr=172.28.108.71, Name=MTA
O DaemonPortOptions=Port=smtp,Addr=127.0.0.1, Name=MTA
Hope this helps.