Editable emails (EMs or EEMs?) would behave like those in the Google Wave platform: individuals can edit within one post or add to the thread in separate posts; changes/revision history and prior snapshots(versions) are all viewable and accessible.
Fixed content emails (FCEs?) would be like PDFs but applying directly to the email rather than an attached file: the content and format would not be editable by recipients.
Editable emails should be the universal standard (eventually the name “editable” would naturally be dropped since most all emails would be this way), allowing for consolidation of mails, better tracking, saving space (that will eventually be a valued commodity, like paper and trees — even though memory will be increasingly accessible and occupy smaller spaces and resources, the most efficient and concise usage and applications of that memory/space will become a natural priority), and especially improved and encouraged collaboration, and much more dynamic communication/interaction.