for a multiple select list to pass multiple values, it must be named with '[]' at the end to create an array:
<select name="myList[]" size="1" multiple="multiple" id="myList">
<option value="1">one</option>
<option value="2">two</option>
<option value="3">three</option>
<option value="4">four</option>
<option value="5">five</option>
<option value="6">six</option>
</select>
to out put the multiple selections in the email, you need to use the implode function to convert the array:
<?php echo((isset($_POST["myList"]))?implode(", ",$_POST["myList"]):"") ?>