This post provides a brief explanation of how the body of a message (HTML in particular) is handled by the different versions of Outlook (Outlook 97, Outlook 98, Outlook 2000, Outlook XP and Outlook 2003) and Exchange (Exchange 5.5, Exchange 2000, and Exchange 2003). We got a question related to this in one of our earlier postings.
For the Exchange server:
Exchange 5.5 doesn't support HTML messages and can only support clients that use the rich text (stored on the server under PR_RTF_COMPRESSED) representation of the body.
Exchange 2000 can handle both Rich Text and HTML messages and can convert between them when asked by clients.
Exchange 2000 also has a feature that determines the best body of a message and returns the best body when requested by a client.
Exchange 2003 is the same as Exchange 2000 except it has added capability for security filtering of HTML which is primarily used by Outlook Web Access. This feature was to remove potentially dangerous content from HTML, which could create virus or do other harm.
For the Outlook client:
Outlook 97 doesn't support retrieving the body from the server as HTML and will only ask the server for the Rich Text format representation of the body. This means that it will always ask the Exchange server for the Rich text format (PR_RTF_COMPRESSED) of a message and any conversion to a different format (text/HTML) will occur on the client.
Outlook 98 started supporting HTML bodies and it can handle retrieving both the HTML (PR_HTML) and Rich Text (PR_RTF_COMPRESSED) representations of the body. It will ask for the HTML body only if it determines that the server supports it otherwise it will ask for the Rich Text body.
Outlook 2000 behaves same as Outlook 98.
Outlook XP is capable of using the best body feature of the Exchange server. The best body feature can only be used when in online mode and won't work properly for offline mode.
Outlook 2003 supports best body feature in both online and offline modes. The feature is also enabled for cache mode resulting in the same body handling logic for both PST and OST. Other than those, it works the same as Outlook XP.
The converter library (used for doing client/server side conversion between HTML/Plain Text/Rich Text) is different in each Outlook/Exchange version and as such the output can change slightly between Outlook/Exchange versions.