You may have a corrupt .dbx file in Outlook Express. See this site for a fix.  Also make sure that you do not have Hide Read Emails checked. You can find this by opening Outlook Express, going to View>Current View and make sure there is not a check mark at Hide Read Emails. 
I am having problems with my outlook express mail. When I open my e-mail it shows my new mail, then when I navigate or close it the mail disappears from my inbox? I hope I am at the right forum for someone can help me?
Outlook Express