Sort Custom Post Type by Custom Field

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How can you sort a custom post type by a custom field in WordPress? I’m trying to figure out a way to echo a custom field called ‘State’ alphabetically and place the custom post type links called employees under each of their respect states.

So it would look like this:

Alabama
Appleseed, Johnny
Chamber, Pete

Florida
Miller, Sam
Stark, Amy

Virginia
Crites, Sue
Lake, Mary

I know how to write the basic loop for a custom post type but I’m not sure how to sort under a custom field belonging to that post type alphabetically.

<?php $query = new WP_query ( array( 'post_type'=> 'Donor', 'meta_key'=> 'email', 'orderby'=> 'meta_value', 'order' => 'DESC'));
  while ( $query -> have_posts() ) : $query -> the_post(); ?>
        <?php echo get_post_meta($post->ID, 'email', true); ?>
    &lt;h2&gt;<a>"&gt;&lt;?php the_title(); ?&gt;</a>&lt;/h2&gt;
  &lt;?php
endwhile;   
wp_reset_query();  ?&gt;

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