Message type conversion is processed instantly, before any rules are applied. You can set up ChangeMessageClass on per-mailbox basis or as store-wide application, i.e. it will work on on each user's Inbox in the certain Exchange 2000/2003 Mailbox Store. The application is server-centric, so you do not need to install any extra software on the client side.
Requirements:
Microsoft Exchange 2000/2003
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