Where exactly the system is picking up the Content type=Text/HTML or Text/Plain while sending EMAIL

Hi Team ,

    I have a requirement to send an email with text substituted value and with an HTML link  .

    So i am using System function SEND MESSAGE EXTENDED  to send an  email by choosing SMTP address (as suggested by business we do not have An8 value , only email address is there.)

Problem :

          For text substituted value we used Glossory group DD item  with HTML tags . But the HTML tags are not working, it's  comming as plain text .

Observation :

          For  my testing purpose i used System function SEND MESSAGE EXTENDED by  passing An8=1234(eaxmple) . here HTML tags which i used inside DD item is working fine  .