[Dovecot] Global sieve filter ignored (Long)

[Larry Crouch]

Hello,

I built my mailserver around the Ispmail server for Lenny tutorial on 
www.workaround.com
No signicant load as it's just my home network.
I originally used the guide for etch and then upgraded.
Almost two years later I have experienced no problems.

Lately though I considered filtering spam with a global sieve filter.
I created the /var/vmail/globalsieverc file which contains:
require ["fileinto"];
# Move spam to spam folder
if header :contains "X-Spam-Flag" ["YES"] {
fileinto "spam";
stop;
}

Made certain it was owned by vmail.

Changed the last stanza of /etc/dovecot/dovecot.conf which in labeled 
plugin:
sieve_global_path = /var/vmail/globalsieverc

Restarted dovecot

Sent a known spam email (GTUBE)

The resulting email is always delivered to INBOX:
2010-07-19 03:35:19 deliver(xxx at xxx.xxx): Info: msgid=: saved mail to INBOX

What am I doing wrong?