Can't send to mailing list from server

When attempting to post to this list, my sendmail records the following
message in maillog:

Sep 8 10:31:12 brightstar sm-mta[30272]: i88F7EQV030166:
to= , ctladdr=
(1003/1003), delay=00:23:57, xdelay=00:00:06, mailer=esmtp, pri=120807,
relay=mx1.freebsd.org. [216.136.204.125], dsn=4.2.0, stat=Deferred: 450
Client host rejected: cannot find your hostname, [209.x.y.z]

209.x.y.z is a static address with a DNS entry courtesy of my ISP.

Any idea what the list's mta is looking for and how I can provide it?

Thanks

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