Alexander Pushkarev

$letter = current_letter();
add_filter('posts_where', function($where) use ($letter) {
    return $where .= " AND post_title LIKE '".$letter."%'";
});

add_filter('posts_where', function($where) {
    $letter = current_letter();

    return $where .= " AND post_title LIKE '".$letter."%'";
});

$to = 'sendto@example.com';
$subject = 'The subject';
$body = 'The email body content';
$headers = ['Content-Type: text/html; charset=UTF-8'];

wp_mail( $to, $subject, $body, $headers );