http://www.ietf.org/rfc/rfc2442.txt
This document defines a MIME content type suitable for tunneling an ESMTP [RFC-821, RFC-1869] transaction through any MIME-capable transport. This type can be used for a variety of purposes, including: Extending end-to-end MIME-based security services (e.g., [RFC-1847]) to cover message envelope information as well as message content. Making it possible to use specific SMTP extensions such as NOTARY [RFC-1891] over unextended SMTP transport infrastructure. Enabling the transfer of multiple separate messages in a single transactional unit.