Il faut modifier le script horde/imp/config/mime_drivers.php en faisant

root@luxy:~# diff -Naur /var/www/horde/imp/config/mime_drivers.php.orig /var/www/horde/imp/config/mime_drivers.php
--- /var/www/horde/imp/config/mime_drivers.php.orig     2008-01-19 10:48:31.000000000 +0100
+++ /var/www/horde/imp/config/mime_drivers.php  2008-01-19 10:55:16.000000000 +0100
@@ -56,14 +56,14 @@
 /**
  * HTML driver settings
  */
-$mime_drivers['imp']['html']['inline'] = false;
+$mime_drivers['imp']['html']['inline'] = true;
 $mime_drivers['imp']['html']['handles'] = array(
     'text/html');
 $mime_drivers['imp']['html']['icons'] = array(
     'default' => 'html.png');
 /* If you don't want to display the link to open the HTML content in a
  * separate window, set the following to false. */
-$mime_drivers['imp']['html']['external'] = true;
+$mime_drivers['imp']['html']['external'] = false;

 /**
  * Image driver settings