I have set up ScrollOut F1 in front of GroupWise.
It works quite well, and I've seen a definite decay in spam compared to our previous solution.
I have one problem though.
I've asked GWIA to mark mails as spam according to a line in the header.
I've set this single line in GWIA Junk Mail:
X-Spam-Flag: YES

When a spam is detected I see the following lines added to the header:
X-Spam-Flag: YES
X-Spam-Score: 5.182
X-Spam-Level: *****

I've also activated Junkmail handling in Client options.

But when spam arrives I don't see it being moved to the spam folder.