More often than not, KMail in recent times tells me that HTML mails are just plain text messages, even though they show correctly in other Mail clients and I can see the HTML part in the mail's source.

=== Example mail source ===
...
Content-Type: Multipart/Alternative;
  boundary="------------Boundary-00=_RC6YVA40000000000000"
...
--------------Boundary-00=_RC6YVA40000000000000
Content-Type: Text/Plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable




[---001:000000:00000---]

--------------Boundary-00=_RC6YVA40000000000000
Content-Type: Text/Html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

...