Re: [ietf-smtp] [Proposal] confusing parts of the mail system, was 250-MARKDOWN

[Brandon Long <[e-mail]>]

On Thu, Jan 17, 2019 at 5:41 AM Ned Freed <[e-mail]> wrote:

> > On 17/01/2019 04.29, Ned Freed wrote:
>
> > > Clearly not, as you yourself have noted.
>
> > Microsoft supports BINARYMIME for received mail on port 25.
>
> > This something that, I hear, is very difficult for others to do.  I'm
> > told it's very complicated.  Somehow they have figured out how to make
> > this work.
>
> They have not figured out how to make it work, because it's not possible to
> make it work in general. Transcoding destroys DKIM signatures, period. So
> they
> either:
>
> (1) Reject messages sent with BINARYMIME and signatures, either in all
> cases
>     or in cases where they know they are going to forward, leading to
>     unncessary failures, or,
>
> (2) Accept such messages and trash the signatures, leading to unnecessary
>      failures, and,
>
> (3) Deal with dynamic forwarding cases either by not having any or
>     falling back to (1) or (2), leading to unnecessary failures.
>
> My guess is that the number of BINARYMIME messages they get via SMTP is
> so tiny they haven't had to deal with the problems. And of course since
> the people attempting to send them BINARYMIME encounter problems they
> probably give up on the extension.
>

Um, Exchange trashes signatures on forwarding even without BINARYMIME.
They've been
trying to fix this for years, and only recently made it sorta work for very
specific forwarding
situations.  They re-write multiple headers to be written in their style,
including ones in the
"body" section from a DKIM perspective.

Brandon