Can not open new Netscape email

User never deletes his email. He has 5900 messages in his inbox. Starting yesterday, new mail that arrives and is downloaded to his local folders do not open correctly. When the new messages are opened, it displays the contents of an old message (which doesn’t even exist no less). It is as if the new messages are replaced with the body of a different email. I guess this happened in the past, before I took over, and as a work around, they created a new local folder, moved all mail out of his inbox to this new folder. After doing this, the new messages that came in to the inbox were unaffected and opened correctly. I’m not big on work arounds. Has anyone else come into this problem before? Is there a FIX for it?