http://www.ietf.org/rfc/rfc1428.txt
Protocols for extending SMTP to pass 8bit characters have been defined [3] [4]. These protocols require that messages transported by the extended SMTP are to be encoded in MIME [1] [2]. Before work began on these protocols, several SMTP implementations adopted ad-hoc mechanisms for sending 8bit data. It is desirable for the extended SMTP environment and these ad hoc mechanisms interoperate. This document outlines the problems in this environment and an approach to minimizing the cost of transition from current usage of non-MIME 8bit messages to MIME.