Note the need to use PRE or the equivalent to protect the headers.
Probably additional escaping of the headers are required and likely the
charset needs very careful handling.  Lots of corner cases here,
including what to do with multiple HTML parts.  


    At the moment, I have three versions of forward implemented, one
that sends the entire message as a RFC822 attachment, one that forwards
just the text part, and one that forwards just the HTML part.  The later
two work correctly when the entire message is one part.  I find each
useful in different circumstances so probably we don't want to support
only one of them.

- Mark