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