I was working on a pre internet protocol stack in the early 1980s. It was called the "coloured book protocols" because each application and layer had a different colour spine. Email was "grey book mail" and in truth it was substantively the same as what we all know as rfc822 format. Sent over x.25, store and forward mechanisms up at application layer.

Obviously we looked at other protocol stacks. Like IP.

A colleague said to me one day "how can they call it 'simple mail transfer protocol' when it's a quarter inch thick pile of fanfold paper" pointing at the 132 column line printer printout on my desk of SMTP.. (which in turn was rfc821 at the time)