It turned out that OSIRUSOFT had gone belly-up, and started declaring that "the world consists of spammers"..... or something.
Digging into the facts gave me the following link:
<http://news.spamassassin.org/modules.php?op=modload&name=News&file=article &sid=44&mode=thread&order=0&thold=0>
and the short fix is this (for people who know how to manipulate SpamAssassin scores):
score RCVD_IN_OSIRUSOFT_COM 0 score X_OSIRU_DUL 0 score X_OSIRU_DUL_FH 0 score X_OSIRU_OPEN_RELAY 0 score X_OSIRU_SPAM_SRC 0 score X_OSIRU_SPAMWARE_SITE 0
Shows the risks of depending on too many services you don't track the status of.....
Harald