In Exchange 5.5, when you emailed a MIME message to a public folder, it would be delivered to the public folder as an email message (message class IPM.Note). In Exchange 2000/2003, this behavior changed, a byproduct of the new support for native MIME content. In Exchange 5.5, all messages were converted into MAPI, even if they originated as MIME. Starting in Exchange 2000, MIME messages are stored natively (and certain properties are promoted into MAPI so that Outlook clients connected to the Exchange server could read & manipulate the messages).
In Exchange 5.5, an inbound MIME email would be delivered first to a private folder where it received the default message class of IPM.Note, and then it would be delivered to a public folder where it maintained that message class. In Exchange 2000, the stores were capable of storing native content, and so MIME messages would go directly to the appropriate store without conversion, where they picked up the default message class for that store (and the default message class for the public store is IPM.Post).
In Exchange 2000/2003, email messages from Outlook clients inside the organization (or TNEF'd messages from outside the org) would be delivered as IPM.Note even to public folders, because in that case the message class of IPM.Note was already set on the message (encapsulated in the TNEF) and there was no need to have the message assume the default message class for the type of store.
Fortunately, there are hotfixes for Exchange 2000 & Exchange 2003 that change this behavior, so you can have your internet mailing lists be delivered to public folders and maintain the ability to reply and see headers. See this KB article for more information: http://support.microsoft.com/?kbid=817809. KB #292484 also has some workarounds.
 If the MAPI client modifies the message, it will then be fully converted into MAPI. I wrote an article about native MIME in Exchange 2000 a few years ago that goes into detail if you're interested. Naresh also touches on this area in the history of content conversion in Exchange.