in the last days there were more and more SPAM mails where I found nobayesian scoring in the header. This lets me guess it did not take place atall. Is that conclusion right?
They may not have been scanned, but then you should see no SA score at all.A better chance is that they scored bayes_50, whihc means "I don't know". Since it is a useless score with a value of zero, some versions of SA don't show this result.
I'd try training some of them as spam and see if they start showing up. Loren