The post is outdated, please read this one https://alltime.pp.ua/blog/how-to-enable-inline-html-display-in-horde/
Edit the following file
vi /usr/local/cpanel/base/horde/imp/config/mime_drivers.php
Replace the line :45 from:
$mime_drivers['imp']['plain']['inline'] = false;
to
$mime_drivers['imp']['plain']['inline'] = true;
Done!