Internet Mail Architecture
Over its thirty-five-year history, Internet Mail has changed significantly in scale and complexity, as it has become a global infrastructure service. These changes have been evolutionary, rather than revolutionary, reflecting a strong desire to preserve both its installed base and its usefulness. To collaborate productively on this large and complex system, all participants need to work from a common view of it and use a common language to describe its components and the interactions among them. But the many differ
Informational, not a standard. Published for the record and carrying no standards weight. Worth knowing before citing it as a requirement.
What it is
A map of the whole email architecture and the vocabulary for it: MUA, MSA, MTA, MDA, and the boundaries between them.
What it solves
Every other document assumes the terms. Nothing defined them in one place, so discussions talked past each other.
What it means to operate
Not a protocol and it has no requirements, which is why this page lists none. It is the document to read first if the handoffs are unclear, because most arguments about whose problem a failure is come down to which boundary it crossed.
Normative requirements
This document does not use the RFC 2119 keywords. It states its requirements in ordinary prose, which is why nothing is listed here rather than because nothing was found.