Excel attachment being sent from an Outlook 97 client is being received at some but not all Outlook Express clients. In fact the data is there if I look at File properties but it does not appear as an attachment. Encoding on sending and receiving PC's is set as MIME.

What should I check in the setup of Outlook Express? Is it a version issue as the only ones not receiving the attachments are running OE 5.00 rather than OE 5.5, how do I upgrade to the latest version?