email via cups-pdf

I just upgraded from 10.5 Server to 10.6 server, and now I’m seeing errors related to email via cups-pdf. I’m using cups-pdf to create a pdf, and using a cups-pdf-dispatch script to post-process the resulting pdf, including sending an email. This was working fine with 10.5, but the email part is broken with 10.6 (everything else works perfectly). I’m seeing these errors in the SMTP log:

postfix/sendmail[41466]: fatal: execvp /usr/sbin/postdrop: Operation not permitted
postfix/sendmail[41465]: warning: command “/usr/sbin/postdrop -r” exited with status 1

I assume there is a permission problems somewhere related to an unprivileged user, but don’t know where to start. The permission on postdrop are:

-rwxr-sr-x 1 root _postdrop 474K Dec 3 15:07 postdrop*

which appear to be usable by everyone, so the above error is mystifying. These are the permissions on the cups-pdf-dispatch script:

-rwxr-xr-x 1 eterr staff 7.4K Aug 24 08:49 cups-pdf-dispatch*

Any clues? Thanks.