Sending email to hidden field using webform after calling hook_form_alter?
The problem is probably caused by the hidden field having the #value property set which overrides #default_value (even if the former is NULL or a empty string).

To get the desired result you have 2 options:

1) Override #value instead of adding a #default_value:

$form['submitted']['lt_email']['#value'] = $_SESSION['Some']['Email'];


2) Unset #value then add #default_value:

unset($form['submitted']['lt_email']['#value']);
$form['submitted']['lt_email']['#default_value'] = $_SESSION['Some']['Email'];


If you are wondering which option is the right one for you then #default_value means that Drupal will accept client-side changes to the field value (via javascript for example). If you don't need/want that possibility then #value should be used (Drupal will ignore any client-side changes in that case). It's mentioned in the documentation as well.