Archived:Replacing e-mail viewer on Symbian (Known Issue)
Replacing the built-in e-mail viewer application and graphical message icons with another application is not possible for the same message type.
You can only create your own viewer/editor if you also write completely new MTM components (server, client, UI, and UI Data). However, this solution would not be usable for other e-mail services than those pre-defined in your code. A more general solution would require a settings editor to be integrated into the MTM UI. This settings editor then has to be able to create new and delete old services for your protocol (MTM) type.