If you see these messages in your syslog, your sendmail is not configured correctly with the domain name. If you are sending from a fully qualified domain, add it to the local-host-names file. In Ubuntu, it would be /etc/mail/local-host-names. Make sure it reads something like this.
Now in the /etc/hosts, make sure it reads the following.
127.0.0.1 [domain.com] [server name] localhost
sudo /etc/init.d/sendmail restart
and the error should not continue filling the syslog.