MIME introduces a structured Internet message format. It can represent various data formats and various coded character sets. MIME thus represents an important part of Internet message processing. Conventional message user agents (MUAs) which process Internet messages have different characteristics. The quality of the user interface depends on the user's preference or habits. In this regard, it may be preferable to allow the user to select a good combination of the MUA and a separate MIME module. This paper describes the architecture of a general MIME processing module to provide its features for various conventional MUAs. In the design of such a module, two complementary models are proposed: "representation layer model" which handles complex structured data and the "acting situation model" which processes various kinds of data under various environments.