This function is not working in 4.10.3 
set your KMail to not prefer HTML rendering. Set a contact to be allowed html rendering. Works
then set KMail to prefer html rendering . Said contact will now be rendered "plain text" despite Kaddressbook contact setting to HTML. 
The addressbook setting works as "default / not default" - not as stated "default/plain/html"

Reproducible: Always

Steps to Reproduce:
1.Set KMail to render incoming mail as plain text
2. Set one contact (or several) to have HTML rendering allowed. This works
3. Change KMail rendering to prefer HTML
4. the contact you set to HTML rendering will switch to plain text rendering
Actual Results:  
see above, self explanining

Expected Results:  
the setting in Kaddressbook of "Default/Plain text/HTM" should work as intended.

As it is now, the Kaddressbook/contact setting works as "default/non default", regardless of what option you set besides Default.