If your mail disappears as soon as you've read it, it's probably because you're using a read-only view. The default Outlook setting for the preview or reading pane is Mark as read after 5 seconds and when the view refreshes the message is hidden. It's less of a problem with Outlook 2003 because the view isn't refreshed until you leave a folder and return or press F5.

If you're using Exchange and you see the message arrive in the Inbox and it immediately disappears, you likely have a *.pst configured as the default delivery location. With the *.pst set as the default delivery location, the messages are moved out of the mailbox and into the *.pst as they arrive in Outlook. Verify the profile is using the Exchange mailbox as the message store not a personal message store. Look for this setting in Tools, Email accounts, View or change existing email accounts. It's at the bottom of the Account manager dialog. In Outlook 97/98/2000, look for account properties using the Tools, Services dialog.

If the delivery location is correct and you don't have another profile open on another computer getting you email, or aren't using an Exchange mailbox, check the rules in Tools, Rules Wizard. Disable all of the rules and re-enable them one at a time, verifying they work as expected.

Both Exchange and POP3 accounts can have errant rules moving messages to another folder. While rules are less likely to cause a message to disappear right before your eyes, rules are responsible when you see or hear the new mail notification alert but discover there isn't new mail in the Inbox.

If the email server uses a content control or anti-spam scanner and mail you're expecting doesn't arrive, check for quarantined messages. In most cases the messages are quarantined before they make it to Outlook, but some scanners can be run offline, (i.e., after the mail is in your Inbox). While the junk email feature in Outlook 2003 doesn't have a "run offline" feature, the junk email filter scans all the email in all Exchange mailbox folders when you create a new cached mode message store, moving suspected junk email to the junk email folder. So if you're using Outlook 2003 and are missing email after creating a new *.ost, look in the Junk email folder.