I am having the same problem. To rephrase it:
Cascading list box correctly displays information (multiple lines of information). However, upon submiting my form to an email address, the list box control appears, but is empty. If I were to select a line in the listbox before I click Submit, it would show that particular line in the email... but you are only able to select one line at a time in a listbox.
I need to email the entire contents of my listbox. I have tried using a textbox in lieu of this, but can't get a textbox to display information from a datasource.