Setting thunderbird labels with procmail

Hi all,

I’m using procmail to filter my mailing list mail into seperate IMAP mailboxes. I’d like to label some of these so they will appear in a different client in thunderbird, using procmail rules. I’ve see references to X-Keywords, but the following doesn’t seem to work:
:0 fh w

The X-Keywords is edited in, but it’s not honored. Does anyone have any suggestions as to how to do this?