I don't get why plain text would be more accessible than a well-formed HTML email. A screen reader could say "this is a title", "this is a list item".
In plain text, what do you get? "dash bla bla"? Though I hope screen reader are more intelligent than that, but it seems like a more complex problem than just using information given by a well-formed HTML text.
Give people HTML and most e-mails will be a complete undecipherable garbage on the markup level.
Basically almost everyone puts anything there so that they get some visually pleasing result, the markup semantics be damned.
One would think that lack of features of various mail clients would lead to using the lowest common denominator of some basic tags like b, i, a, hr, and maybe img.
Not so in reality.
One bank I've seen e-mails from even abuses invalid parsing of HTML comments, to produce whatever insane result someone somewhere dreamed up, like this:
<!--><div style="some:crap"><!-->...
Which is straight up invalid HTML, that sanitizing parsers like caja will reject. I had a fun of telling a client, that I will not purposefully break parsing algorithm of a HTML sanitizing parser just so that their customers can read mails from their bank.
(Experiences from writing a web mail client to be used in a real world.)
Well, plain text doesn't require smart or sophisticated screen readers. Anything that can read text will work, that's why plain text emails are more accessible. Not every person that needs screen reader can afford some top of the line device.
In plain text, what do you get? "dash bla bla"? Though I hope screen reader are more intelligent than that, but it seems like a more complex problem than just using information given by a well-formed HTML text.