Found in a french forum:
Paste
<?php
require_once(‘mp-includes/class/MP_uninstall.class.php’);
new MP_uninstall(); ?>
into uninstall.php as first line.
Trying to uninstall from within wordpress (MP 5.1, WP 3.2.1)
Fatal error: Class ‘MP_uninstall’ not found in […]/wp-content/plugins/mailpress/uninstall.php on line 1
What do I do now?