*mime_alternative_prefs: text/plain text/richtext text/html
which says, given a a multiplart alternative, preferentially pick
the text/plain first, then text/richtext, then text/html. This
violates the RFC of course, but is also rather user friendly.
In what way does this violate the RFC? RFC2047 reads:
Systems should recognize that the content of the various parts are
interchangeable. Systems should choose the "best" type based on the
local environment and references, in some cases even through user
interaction. As with "multipart/mixed", the order of body parts is
significant. In this case, the alternatives appear in an order of
increasing faithfulness to the original content.
In other words, which part you choose to display is purely a local matter.
There's nothing anywhere that says that the ability to display the "most
faithful" version requires you to display it.