http://www.ietf.org/rfc/rfc2646.txt
Interoperability problems have been observed with erroneous labelling of paragraph text as Text/Plain, and with various forms of "embarrassing line wrap." (See section 3.)

Attempts to deploy new media types, such as Text/Enriched [RICH] and Text/HTML [HTML] have suffered from a lack of backwards compatibility and an often hostile user reaction at the receiving end.

What is required is a format which is in all significant ways Text/Plain, and therefore is quite suitable for display as Text/Plain, and yet allows the sender to express to the receiver which lines can be considered a logical paragraph, and thus flowed (wrapped and joined) as appropriate.

This memo proposes a new parameter to be used with Text/Plain, and, in the presence of this parameter, the use of trailing whitespace to indicate flowed lines. This results in an encoding which appears as normal Text/Plain in older implementations, since it is in fact normal Text/Plain.