The implementation of the WSO2 IS 5.3.0 and below doesn’t render the HTML content (text/html) instead, it sends email notifications as plain text (text/plain). This is because it uses the Axis2 Client (org.apache.axis2.client) to send the emails and it is a limitation of the Axis2 Client.
This project is capable of rendering the HTML content as it uses the javax.mail library and this can activated using five simple steps for the WSO2 IS 5.3.0 and below.
WSO2 IS 5.3.0 and below
Please refer [1] to activate it.