Probably you use get_post
on edit page with query object. You can simply use $post->post_author
on edit page. For example;
$editableid = get_query_var('id');
$postedit = get_post($editableid);
$author_id = $postedit->post_author; //Always return author ID value.
<input type="hidden" name="post_author" value="<?php echo $author_id;?>" />
I hope this code piece helps you.
EDIT : For email you use the_author_meta with your $author_id
value.