error when users try to unsubscribe

I have a few WP sites that use this plugin and it seems to function fine until someone tries to unsubscribe. They receive the following error.

Warning: Cannot modify header information - headers already sent by (output started at /var/www/wordpress/wp-content/plugins/subscribe-by-email/subscribe-by-email.php:262) in /var/www/wordpress/wp-includes/pluggable.php on line 934

I have tried subscribing and then unsubscribing myself from 2 of our sites and get the same error. We are running WP 3.2.1 and I have tried replacing the subscribe-by-email.php file with a new one and have also deactivated and reactivated the plugin.