I use AirWatch Agent and created an ActiveSync account to connect to my company's exchange server. Now, when I read emails they show up as plain text with the message "security policy restricts use of html email". I've tried going over settings to find something related and found none.
I'm willing to bet that you can't change it. It's probably being enforced by AirWatch as part of your company's security policies. They have disabled HTML email; so that means HTML email will be converted to plain text.
I came across this issue during the deployment of AirWatch.
We had to reinstall the profile on the secure email gateway server. Once that was done the policy to allow or deny HTML email appeared on the profile.