Sometimes (I haven't been able to figure out what triggers it) some of my emails don't show up in the Inbox. The send/receive dialog box shows the correct number of incoming messages but they don't all show up in the inbox. For example, the send/receive dialog box may say I am downloading three messages, but the inbox (and the number of messages displayed by the "inbox" folder) only shows 1 new message. If I change to any other folder, (sent items, for example) the number of messages beside the folder instantly updates to 3 and when I go back to the inbox the missing emails are there.
I originally installed Outlook 2003 on this clean-installed Windows 7 PC. I then later upgraded to Outlook 2010. There is no Exchange server involved... I am downloading messages from both my ISP and another mail provider hosted at Network Solutions. Since the send/recieve dialog box always correctly identifies the number of incoming emails, I'm assuming the problem is local to Outlook. There are no add-ins and only a couple of rules.
Googling this problem turns up a handful of people with the exact same issue (in various versions of Outlook) but I haven't seen a fix.