If you are heavy email user then to loose some of your messages can be disastrous. In many cases the email is not "missing" you just cannot see it or it has been moved elsewhere.
If you are using pst files (Personal Folders) then you need to be aware of that these are limited by design to 2gb. Keep a close eye of them and move to a new one when you get to between 1.5gb and 1.8gb so that you don't loose any messages. If the file goes over this size then Outlook cannot open it. While there are tools to shrink the file, all they do is cut off any data so that the file become small enough.
If you delete a lot of messages from your personal folders file, then you should compress it. This is because Outlook just leaves the space behind without filling it.
Q: Some of my email has suddenly disappeared. Where is it and Can I get it back?
A: Your machine has probably archived it. Outlook is set up by default to archive all email over six months old to a separate folder.
To check whether Auto Archive is enabled, click on Tools, Options, then the "Other" tab. One of the buttons is labelled "Auto Archive". Click on it. If the option at the top is selected then the auto archive feature is enabled. By default it archives all email that is older than six months automatically.
However, before you disable the feature, you need to make sure this is what has happened. About half way down you will see "Move old items to" and a path name. Close the options dialogue and then click on File, Open, and Choose "Outlook Data File" or "Personal Folders" (label depends on the version of Outlook you are running). Then browse to the path that you have just seen above. When you open the file, you should see your old email. If so, you will need to copy it back. Old Calendar and other items will also be stored in there.
Once you have copied them back, turn off Auto Archive in the Tools, Options, dialogue that you were in before.
Q: I have deleted a message in error, and emptied my "Deleted Items" folder. Can I get it back?
A: Possibly. There is a feature on the server known as "Deleted Items Retention" which means that any email you have deleted from the "Deleted Items" folder hasn't been deleted off the server. This period is set by the email server admin. Check with your local admin for information on what their setting is. More Information.
Q: I cannot see all of my messages.
Q: My messages seem to disappear as soon as they arrive.
A: The most likely cause of this is either filters (or rules) or views.
If it is views, you might have Outlook set to only show certain messages - perhaps from a particular person or on a subject. Click on "View" and choose "Messages" to see all of your messages.
If you are using an Exchange server then you can also use OWA to see whether your email is there, as it is not affected by views, as these are a client side setting.
The other cause could be a filter or rule moving the message to another folder. Check your rules in "Tools", "Rules Wizard". You can turn off the rule by unselecting it from the list. You don't have to delete it.