As I said, I don't know of any way to do that. Even manually, the only way to do it is to copy and paste.
Is Exchange your mail server? If so, then application B could in theory read and write directly to the Exchange Server without involving system A at all.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers