thinking the situation over one more time i would like to ask the following;
regarding that Windows Live Mail is not supported and therefore not installing a plugin for spam;
is it possible that the mailclient Windows Live Mail itself is classifying currently incoming mail for spam ?
or you Internet Mail provider perhaps ?
or did you have an other AV program on the computer than McAfee and remains of that other AV program are left behind and causing the problems perhaps ?
are the above mentioned suggestions perhaps a possibility that could cause the current problems ?
No, Microsoft Live Mail (which has had no updates in at least a decade) did not suddenly start trying to detect and mark spam messages
No, neither of the two different mail providers involved are marking the mail as [SPAM] -- both have a spam filtering option, but both those options are turned off, and neither marks spam emails this way.
No, there is no other anti-spam or anti-virus program running on the computer.
And, finally, if I didn't already make this clear, not only did the problem only start when McAfee Small Business Protection was installed, but the emails that were marked as [SPAM] each had an added header showing that McAfee had processed the message.