Outlook can become unstable if your Inbox grows too big.
So what can you do? Fortunately, Outlook offers a few key ways of keeping your inbox lean and mean, while still allowing you to access all of your e-mail quickly, back it up regularly, and control where you store it

• First Step: What's the size of your Outlook inbox file?
  1. Open Outlook
  2. Click on your inbox folder
  3. Right-click and select PROPERTIES
  4. On the bottom of the GENERAL tab, you can view FOLDER SIZE
     (NOTE: If you're running Outlook on your PC on an Exchange server you can see the size of the file on your PC and compare it with the size of the e-mails you have sitting on the server)
  5. Do the same with your SENT messages.
• Organise your folders

  Keeping all the relevant e-mail messages in the Inbox is only workable when you only receive a few e-mail messages a day. The Inbox might contain hundreds of messages, but a quick visual search or a search with Find or Advanced Find would quickly turn up any required messages.
  As e-mail use grows that system becomes unworkable. The Inbox would regularly contain over 2000 messages. Even with Find or Advanced Find, finding anything in that mess took a while. Searches might turn up dozens of matches that would need to be looked through to find the information needed.

  The answer to the problem was to create a system of folders to hold related messages (and other Outlook items as necessary).
  To create a new folder, follow these steps:

  1. Click File, New, then Folder. This opens the Create New Folder dialog box shown below.
  2. Creating new folders in Outlook makes it possible to manage your information more efficiently.
  3. Enter the name for the folder, making sure to choose a name that will remind you of what is in the folder later on.
  4. In the Folder Contains list, select the type of Outlook items that you plan to store in the folder.
  5. In the Select Where To Place The Folder list, select the name of the folder that you want to contain your new folder. For example, select Inbox if you want the new folder to appear as a subfolder of the Inbox.