Error upgrading postfix on debian

When upgrading my mail server this morning, I got this error message:

 insserv: script postfix: service mail-transport-agent already provided!
insserv: exiting now!
update-rc.d: error: insserv rejected the script header
dpkg: error processing postfix (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 postfix

If you run into this, grep /etc/init.d for "mail-transport-agent" and find the associated file.  "insserv" manages the daemon startup files.  It decides if a service is present from specially formatted comments in the init.d files, not from the package database.  In my case, the problem was a stray init file from a long-uninstalled instance of exim4.  "apt-get purge exim4" hadn't removed it.  "rm -f" did :-)