Gavin McTaggart respondeu a uma pergunta semelhante, embora sobre o Exchange 2007:
Have a look at this Technet document, specifically the ImapMessagesRetrievalMimeFormat switch. The documentation is exceedingly vague, but Exchange can force the format of the messages as they are retrieved from the server, either by user (as the link indicates) or on the service itself; in which case you would use the Set-ImapSettings command, and change the MessageRetrievalMimeFormat option.
Ainda não tive a chance de testá-lo. Eu realmente não tenho qualquer noção de que o Exchange Management Shell indiretamente referenciado existe ou funcionará com o Exchange 2003, no entanto.