In a perfect world I think we SHOULD parse those messages (up to the
limits of virtual memory), but right now we don't.
My regular statement that no we should not accept such dribble into the
system. Users should be aware it's arriving. If they then want to fix
it, then mhfixmsg(1) is where knowledge of all the world's crud can be
put. And some users, like David, can arrange for it to handle all
incoming email so they never need to know.
I would not like virtual memory to have to be exhausted, crippling the
machine's performance, before nmh finally bails out and gives a `line
too long' that it could have stated much, much earlier.