How do I enable the Trash Folder in Horde for all email accounts by default ?

i am also interesting in it....

In imp/config directory, you will find a file called prefs.php.
Inside the file, you will see something similar in the file.
// should we move messages to a trash folder instead of just marking
// them as deleted?
// a value of 0 = no, 1 = yes
$_prefs['use_trash'] = array(
'value' => 1,
'locked' => false,
'shared' => false,
'type' => 'checkbox',
'desc' => _("When deleting messages, move them to your Trash folder instead of marking them as deleted?")
);

if the 'value' is 0, change it to 1.

what if i am using a reseller hosting....

Ask your provider to do it, or install your own copy.

cool. thanks :)