A feature to append mail or news messages to a file would be great, so that you can save interesting messages into a file in the mbox format. was also requested on npm.wishlist, see http://groups.google.com/groups?as_umsgid=3CA6B9CD.9010700%40nyu.edu
My new NoteTaker! context menu extension allows you to "clip" text to a file when you're browsing, it doesn't work with mail & news yet but... http://code.rancidbacon.com/ mbox, XML etc *may* be a future addtion... :-)
Iiuc, as a workaround at least, one can simply use a local folder (mbox format): copy messages to it (then copy that file wherever you like).
Ooops, should dupe the other way round...