I'm also looking for a way to set the move_junk field on by default.
I tried by editing: /usr/local/ispconfig/interface/web/mail/form/mail_user.tform.php:261
Setting default to 'y'.
'move_junk' => array (
'datatype' => 'VARCHAR',
'formtype' => 'CHECKBOX',
'default' => 'y',
'value' => array(1 => 'y',0 => 'n')
It's simply doesn't work for that field. If i change other CHECKBOX fields on the same page from No to Yes it works. Thats pretty confusing.