Enable update to permit the caching of incoming SMTP messages as IPM.Note in Exchange Server 2003
When you send Simple Mail Transfer Protocol (SMTP) mail to a Microsoft Exchange 2000 Server or a Microsoft Exchange Server 2003 public folder, the message class is IPM.Post.
There is an update that you can use to configure the setting of the PR_MESSAGE_CLASS attribute of SMTP messages that are sent to public folders as IPM.Note instead of as IPM.Post.
After you install the update, the message class of incoming messages to public folders may be changed to IPM.Note (instead of IPM.Post). You can configure this new setting by creating the "Incoming defaults to IPM.Note" registry value.