Thank you for looking into this

I'm not sure what the problem with the message is, I can verify that 
it doesn't render correctly though, unless you have the tidy extension 
installed. This PHP extension cleans up and fixes broken HTML code, so 
that the message displays fine (at least in the latest stable 
version), once it is installed.

Yes, it's still badly outdated though.

Someone from the company that hosts my webmail has replied, saying:
"Checked on my side the exact Horde IMP which is installed on our 
server for your account is Horde (6.1.6)"

Does 6.1.6 make sense as an IMP version number?

Here's a whole raw message that incurs the rendering problem.

I've asked my webmail host about the IMP version, and will get back to 
you with that when they reply.

By the way, I've also raised this with the sender of that email 
message.  I think their META tag looks wrong.  They generate this:
<META http-equiv="Content-Type" content="text/html; charset=utf-16">
...but I think maybe they should generate this instead:
<META http-equiv="Content-Type" content="text/html" charset="utf-16">

If I QP-decode the message body and save it as a .html file, it 
renders OK in my browser.  That's the same browser I use for webmail.

Horde 5.1 is very old, chances are your IMP version is as old, and 
things may have been fixed since then.
Please find out your exact IMP version, and please upload the raw message.

Here's the attachment again.  I couldn't see it in the attachments 
tab; maybe it fell off the form while I was having 2 tries at the 
captcha.

Hi,

Some of my incoming email messages from a particular sender 
(trademe.co.nz) get rendered OK in Mozilla Thunderbird but not in my 
Horde 5.1.5 webmail (hosted by openhost.net.nz).  I couldn't see any 
tickets in your system for 'decoding' or 'quoted-printable', and the 
tickets for 'render' looked unrelated.  I'm unfamiliar with the 
subcomponent structure of Horde, but am guessing that IMP is the queue 
to use because '/imp/' occurs in the URL when I view the unrendered 
email source.

The attached screen shot shows some of the email source, and the 
garbled rendering.  I wonder whether this comes from a bad interaction 
between the charset=utf-16 attribute and the quoted-printable 
decoding.  A web search for 3Dutf-16 charset led to this page - 
http://www.novell.com/documentation/datasynchronizer1/resources/datasync12_bugfixlist_mobility_upd3.html - which suggests that Novell may have had a similar 
issue.

I hope this is useful feedback.  Thanks for reading!