"Blank" emails in Webmail

[#1]

I've previously mentioned that some emails don't properly display in Webmail - ones from the AA for instance. Also some emails don't display at all.
I've attached a screengrab showing a Plusnet Webmail window displaying a blank email from Euro Car Parts alongside the same email viewed through Yahoo email (*) - where it's displaying fine.
Am I right in presuming this is a basic "inbuilt" bug in the Roundcube software, thus nothing to do with my settings, and thus nothing I can do to alter the situation?
The email shows blank either using Firefox on my XPSP3 desktop or Chrome on my W8 laptop so it doesn't appear to be platform or browser dependent - unless the same problem is affecting both.
The email shows correctly when viewed through Windows 8 Mail.
(*  -  In case you're wondering why it's in both places, my incoming emails to [mydomainname].co.uk are forwarded to both PlusNet and Yahoo - something I set up shortly before transferring to PlusNet from Orange owing to some going to Orange being blocked - I changed to avoid the problem but left the redirection to Yahoo in place)

[#2]

Check the email headers for one particular line (which normally is the culprit):
MIME-Version: 1.0
If it's missing then it won't display properly or not at all (depending on what's been written into the HTML code).

[#3]

Yes - that's similar to the comment I had when I reported the AA issue - but with respect if it's working in what I'll call "mainstream" webmail platforms, why doesn't Roundcube work too? It isn't the "fault" of the company issuing the email - it's a deficiency in Roundcube.
Seems to me a prime reason for NOT using Roundcube - isn't it?

[#4]

In fact I find this in the relevant email's header:
MIME-Version: 1.0

[#5]

Hmm... Could you give the full header, removing personal info of course? Not sure what the issue is with that email.
As to RoundCube, the mime version is a required tag in the email standards. I could point you to the standards page but it's a bit difficult to do on this iPod (fiddley having multiple pages open in mobile safari & sometimes loses text inputed due to unwanted reload of the page). Other webmails I guess are probably more lenient and just assume a few things. I suppose the most appropriate comparison is the way Firefox (keeps to standards) vs. IE (does something else, not to standards) displays web pages.

[#6]

Return-path: <bounce-mc.us1_932597.1340033-drj=[my domain name]k@mail49.atl51.rsgsv.net>
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From: =?utf-8?Q?EuroCarParts?= <offers@eurocarparts.com>
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Date: Mon, 9 Dec 2013 14:13:49 +0000
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[#7]

Nothing really untoward there. Possibly RC not reading/recognising the boundary lines correctly but that's just a guess, because to know for sure you'd have to know the RC source code quite well.