If you take a look at /var/log/daily.out, you'll see these error messages repeated there (assuming you've set up your computer to do its maintenance tasks). I don't get these messages when sending mail to root, I get them as part of the daily maintenance tasks or if I issue the 'sudo mailq' command.
The interesting thing is that when I issue the 'mailq' command with the -v (verbose) switch, I get the following message:
Warning: .cf file is out of date: sendmail 8.12.2 supports version 10, .cf file is version 9
Would be nice to see Apple address all this stuff.