The following MUAs have reasonable support for displaying nested
multiparts:
* EXMH 1.6.7
TkRat does show all nested bodyparts, no support for composing them
other than forwarding a message though (yet). One very useful
application of this is that when you reply to a message that contain
nested message/rfc822 (at any depth) it asks you which message you
really want to reply to, this is handy when handling the bounce
messages that newer sendmails generate.
/MaF
PS TkRat is a graphical MUA I am developing the latest available
release is 0.70 which is beta quality. For more information see
http://www.dtek.chalmers.se/~maf/ratatosk/ (unfortunately this
server seems to be down at the moment).