Some Old Messages Are Suddenly Being Moved to Junk Folder

A perplexing problem started a few days ago.  Some (an apparently random few) messages I have read and filed in my Hotmail folders are being moved to the junk folder.  I move the messages back to their original folders, but the next day when I start Windows Live mail, the same messages reappear in the junk folder.  I tried adding the sender to the safe sender list and changing the junk email setting to a lower level, but the messages reappear in the junk folder the next day.  Some of the messages are a few months old and some are several years old.  They were not sent to the junk folder when I originally received them, and they have been filed in their folders for weeks before this problem started.