The "prepend" content filter does not do what (I think) it should do: add text in the "data" field to an email header matching the criteria, or to any header for that matter. I have added the recommended "plugin" script from /usr/share/doc/dovecot-core/sieve/extensions/editheader.txt to /etc/dovecot/conf.d/90-sieve-extprograms.conf Thanks for any help.
