Our approach to supporting PEM across an environment in which intermediate conversions may occur defines an encoding for mail which is uniformly representable across the set of PEM UAs regardless of their systems' native character sets. This encoded form is used (for specified PEM message types) to represent mail text in transit from originator to recipient, but the encoding is not applied to enclosing MTS headers or to encapsulated headers inserted to carry control information between PEM UAs. The encoding's characteristics are such that the transformations anticipated between originator and recipient UAs will not prevent an encoded message from being decoded properly at its destination.
Four transformation steps, described in the following four subsections, apply to outbound PEM message processing: