If you're on Exchange 2000, you'll need to get a copy of the April 2004 Exchange 2000 Server post-Service Pack 3 update rollup (Microsoft Knowledge Base Article: 836488, April 2004 Exchange 2000 Server post-Service Pack 3 update rollup) and install it on your Exchange server(s).
If you're on Exchange Server 2003, you'll need to call Microsoft Product Support Services (PSS) and ask for fix 817809.
Once hotfixed, you'll want to a DWORD registry key named "incoming defaults to IPM.Note" under the following registry key:
HKEY_LOCAL_MACHINE\System\CurrentControlSet \Services\MSExchangeIS\<servername> \Public-<GUID> and set it to "1" to retain the IPM.Note message class.
This is also detailed in my colleague Serdar Yegulalp's article, Public folder message classes.
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