Mail Markup Language (MML) is a simple tag based markup language used
to describe and structure data in a document intended to represent
communication across the medium of email. MML is written in W3C XML
Schema language. All MML documents are XML documents that conform to
the MML schema. This specification intends to replace RFC 5322.
And RFC 5322 states:
This document specifies the Internet Message Format (IMF), a syntax
for text messages that are sent between computer users, within the
framework of "electronic mail" messages. This specification is a
revision of Request For Comments (RFC) 2822, which itself superseded
Request For Comments (RFC) 822, "Standard for the Format of ARPA
Internet Text Messages", updating it to reflect current practice and
incorporating incremental changes that were specified in other RFCs.
And then just Google the subsequent chains, and search for "abstract".