messagelib -- HTML email can open browser window automatically

Albert Astals Cid reports:

messagelib is the library used by KMail to display emails.
messagelib by default displays emails as plain text, but gives the user
an option to “Prefer HTML to plain text” in the settings and if that option
is not enabled there is way to enable HTML display when an email contains HTML.
Some HTML emails can trick messagelib into opening a new browser window when
displaying said email as HTML.
This happens even if the option to allow the HTML emails to access
remote servers is disabled in KMail settings.
This means that the owners of the servers referred in the email can see
in their access logs your IP address.