I upgraded to the latest version of sendmail and now I'm getting these error messages when I run "newaliases":

/etc/aliases: line 13: MAILER-DAEMON... cannot alias non-local names /etc/aliases: line 14: postmaster... cannot alias non-local names

How can I solve this problem?

Your local mailer doesn't have the "A" flag specified. Edit the Mlocal line in sendmail.cf and add "A" to the flags listed after "F=".

Better yet, if you're running a recent version of sendmail that uses m4 to generate .cf files from .mc files, regenerate your sendmail.cf and see if that fixes the problem. Remember to install the new sendmail.cf and restart the sendmail daemon.