Problems with emails arriving in junk & inbox from same sender?

Outlook 2007 – Inbox + Junk mail
Hi, I have a user that has two pcs (in different locations) and obviously they ar eon the network and running Office 2007. On one particular PC the user is finding some mail from senders in his junk and some in his inbox. This is the same person sending mail and it will go, randomly, in either the junk or inbox. Now I ama ware of safe lists etc but it is with a lot of emails that come in. It does not relate to attachments as it happens sometimes with and sometimes without attachments.

I have checked the rules and nothing there either.

Anything I can do apart from adding each time the sender to the safe list?

Thanks