You can enable the html display for horde by using following steps but please make sure that you have been logged in as root.

root@admin[~]# vi /usr/local/cpanel/base/horde/imp/config/mime_drivers.php

Search for the line
—-
/**
* HTML driver settings
*/
$mime_drivers[‘imp’][‘html’][‘inline’] = false;
—–

and replace it with

——–
/**
* HTML driver settings
*/
$mime_drivers[‘imp’][‘html’][‘inline’] = true;
——–
Save the changes and restart the following services.
root@admin[~]#service cpanel restart
root@admin[~]#/scripts/restartsrv_courier