If anybody else is getting hit by SPAM with a URL that contains spaces here is a rule that was posted on the MailScanner mailing list. Seems to work a treat with no FPs.
Code:
# Rule to find URLs with spaces
body ASDM_OBF_URL /www\.\s(.+?)\s[A-Za-z]{2,4}/i
score ASDM_OBF_URL 4.5
describe ASDM_OBF_URL URLs with spaces Either add in /opt/zimbra/conf/salocal.cf.in and restart ZCS or create your own .cf under /opt/zimbra/conf/spamassassin.