How to enable inline HTML email in horde on plesk / odin?

For this you need to go to your favorite editor (nano in my case) and edit a few files.
Here are the files:
/usr/share/psa-horde/imp/config/mime_drivers.php
and
/usr/share/psa-horde/config/mime_drivers.php

In these files you need to find the ‘inline‘ setting under the ‘html‘ key and set it from false to true

Here is a sample how it may look:

/* HTML driver settings */
‘html’ => array(
/* NOTE: Inline HTML display is turned OFF by default. */
‘inline’ => true,

The only downside to this is, that plesk / odin overwrites it every time you install an update.
I’ve solved it with a event in the event manager and each time after an update, i call a php script that sets the values again to true.

I don’t understand why odin/plesk doesn’t include such settings in the main GUI.