dovecot - Mar 2019 - “Message cannot be displayed because of the way it is formatted” iOS

Have updated our mail server and are now running Dovecot 2.3.4. Things are
mostly working OK, but occasionally I have users on latest iOS getting this
message: ?this message cannot be displayed because of the way it is formatted.
Ask the sender to send it again using a different format or email program.
multipart/alternative?

The email seems to open OK on the macOS version of Mail.app.

Also getting ?Loading...? displayed in body section of the app with content
never displayed.

Most emails are fine, but just get this strange ones occasionally. Our previous
mail server was much older, with older version of Dovecot, etc. This server
never used to have these issues.

Just wondering if anyone has any ideas or suggestions? Can?t see anything in the
logs.

Thanks, James.

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On 2019-03-06 12:00, James Brown via dovecot wrote:
> Have updated our mail server and are now running Dovecot 2.3.4. Things
> are mostly working OK, but occasionally I have users on latest iOS
> getting this message: ?this message cannot be displayed because of the
> way it is formatted. Ask the sender to send it again using a different
> format or email program. multipart/alternative?
> 
> The email seems to open OK on the macOS version of Mail.app.
> 
> Also getting ?Loading...? displayed in body section of the app with
> content never displayed.
> 
> Most emails are fine, but just get this strange ones occasionally. Our
> previous mail server was much older, with older version of Dovecot,
> etc. This server never used to have these issues.
> 
> Just wondering if anyone has any ideas or suggestions? Can?t see
> anything in the logs.
You are probably seeing the effects of a bug or shortcut in the iOS mail 
client.

A few years ago, I was working on system that sent bulk marketing 
emails. I had particular problems with iOS and multipart/alternative 
messages. iOS would insist on displaying the second of the multipart 
components assuming that it was html, without actually checking the MIME 
type. For other technical reasons, I wanted to construct the multipart 
mail so that the html version body was before the plain text version. 
Every other email client I tested worked fine, but iOS broke when it saw 
that message.

I also saw problems when iOS mail was configured to only download the 
first 32kb of each email (to save on cellular data), but those problems 
where intermittent and harder to pin down.

In short, I doubt there is a bug in dovecot, it will be following the 
RFCs, but iOS is probably playing fast and loose with the standards.

-- 
David Pottage