How the "reject if possible" is suppose to work was described in this posting by Sigurdur from Linpro (the host for Runbox's mail servers):
http://www.emaildiscussions.com/...203#post160203
Based on that I would have expected your test SPAM to be rejected (assuming you only sent it to one Runbox address).
I think SpamAssassin is handled at the Linpro end so maybe Liz can ask them to comment on this.
Rich