I think before we define a _new_ email standard, that there should be an email standard in the first place. So many clients treat email differently:

* quote levels increasing or not

* how to format the date of the previous message

* whether or not you attach the original files when replying

* how to format the headers signatures of the emails

* where attachments are, i.e. inline or as an extra

* read receipts

on and on.