Outlook 2000 – Error Code

When i am writing a new mail and let the mail stay open for a while, sometimes after a couple of minuste, sometimes after 30 or more minutes, I get the error message “The item could not be saved because it has been changed by another user or in another window. Do you want to make a copy in the default folder for the item”. Why do i get that message and how can i make it never show up again?

Thanks in advance