If you usually send o receive a large quantity of email regularly, it is advisable to follow certain rules to keep the risk of email folders corruption(which is serious because you could not access them anymore),as lower as possible .
Here few easy steps to do the job:

1. Don't use only the default folder Inbox to store all your messages but create new folders with appropriate name and move your incoming messages into them.(You can also Automatically get your messages on the folder you choose by setting Messages rules.To see how click here)
     Steps how to create a new folder:
     a > Open Outlook Express
     b > Right-click the Inbox folder
     c > select New Folder
     d > In the next window just type the name you want to give to
           the new folder in the empty box and press OK
     e > The new folder is created as a sub-folder of the main Inbox
   
2. Keep or try to,any email folder as small as you can.A very big folder could become corrupted
   
3. Same thing for the Sent Items folder,do not wait it's big.A good habit would be empty the folder or,if you can't lose your old messages,back up a copy on a cd or somewhere else on your HD and the you'll be free to empty the folder
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4. Compact your folders once a month. you can see how to clicking here
   
5. Download and install all Windows XP updates.
   
6. Backup all your emails folders regularly.To see how just click here How to Backup email folders