could be possible with jQuery.
This Idea is to add a Hidden field and set value to it with jquery.
$("#name_field_id").focusout(function(){
str = $(this).val();
if(str){
s = str.split(/(?<=^\S+)\s/); //split string in name field after first space found.
$("#hidden_field_id").val(s[1]); //s[1] value after the first space set to hidden field.
}
});
then you can use the hidden field in email.