You need to query the db using wp_query to get all the posts from the CPT, save the metabox contents to an array and then print the array.
Not tested but should work (i hope):
<?php
$select_array = array();
$args = array (
'post_type' => 'your_cpt',
);
$query1 = new WP_Query( $args );
while ( $query1->have_posts() ) {
$query1->the_post();
$select_array[firstname] = get_post_meta(get_the_id(), 'the_meta_key', true);
$select_array[lastname] = get_post_meta(get_the_id(), 'the_meta_key', true);
}
wp_reset_postdata();
?>
<select name="mailing_name" id="mailing_name">
<?php
foreach ($select_array as $option) {
echo '<option value="">'.$option[firstname].' '.$option[lastname].'</option>';
}
?>
</select>
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