To send HTML emails from Horde webclient, please follow the following steps :-
1) Log in to your Horde webmail client.
2) Click the [New message] button.
3) In the "Message Composition" window that opens you will see the option [Switch to HTML composition].
4) Click it and a WYSIWYG editor will open, giving you the option to edit your text's font, size, format, to insert links and images, etc.
5) Once you are ready with the message, click [Send Message] to send it.