Very Computer

The logwatch on my machine has the following entry in its send mail section
that makes me to believe my machine is relaying email from/to third parties
(where some machine uses my machine to send spam to a thir machine).

 --------------------- sendmail Begin ------------------------

Unresolveable or non-existent domains:

Did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA:
    67.17.187.6       :    1 Time(s)

**Unmatched Entries**
   STARTTLS=client, relay=209.145.193.2., version=TLSv1/SSLv3, verify=FAIL,
cipher=RC4-MD5, bits=128/128:
1 Time(s)

 ---------------------- sendmail End -------------------------

I would send and receive email at my machine directly, so far with
apparently no problem, but I want to halt any relaying from other machines.
I feel I haven't really understood the instructions in
/etc/mail/sendmail.mc because I've noticed I haven't uncommented the
following line, and I still do get email from network.

dnl #
dnl # The following causes sendmail to only listen on the IPv4 loopback
address
dnl # 127.0.0.1 and not on any other network devices. Remove the loopback
dnl # address restriction to accept email from the internet or intranet.
dnl #
dnl # DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

I also have the following lines in my /etc/mail/sendmail.mc:

...
...
dnl #
dnl # FEATURE(`accept_unresolvable_domains')dnl
dnl #
dnl FEATURE(`relay_based_on_MX')dnl
dnl #

What (other) lines should I change to make sure mail directed to my machine
does get through, but mailk directed to a third machine doesn't?

Thanks.