By default inline HTML is disabled in Horde.
Resolution
To enable inline HTML, set option “inline” to “true” in the following files:
/usr/share/psa-horde/imp/config/mime_drivers.php
/usr/share/psa-horde/config/mime_drivers.php
Example:
/* HTML driver settings */
‘html’ => array(
/* NOTE: Inline HTML display is turned OFF by default. */
‘inline’ => true,