Publish problem after activating plugin

Hi,

I am using WordPress as a kind of CMS for my website, I use the comment function not as a comment-system but I use it as a guestbook.
For this purpose I want to change the mail that is being send when someone leaves a comment, so in my case a new entry to the guestbook.
I want to change the text and I want it to be send to another mail-address then the mail address of the admin.

After a little search I found out that this mail-function is implemented in the file pluggable.php in the function wp_notify_postauthor. I found out that you can override pluggable functions with a plugin, so I thaught it would be the best solution to write my own simple plugin for this.

I never wrote a plugin before, but with some help from several websites I thaught I managed. I wrote the next plugin to override the wp_notify_postauthor function:

<?php
/*
Plugin Name: Guestbook Notifier
Description: this plugin changes the mail that is being sent when someone writes something in the guestbook
Version: 0.5
Author: MaartenN
*/
?>

<?php

if ( !function_exists('wp_notify_postauthor') ) :
function wp_notify_postauthor($comment_id, $comment_type='') {

	$guestbooksender	=	'blah@blah.com';	//the address that will be seen as the sender of the mail
	$guestbookreceiver 	= 	'blah@blah.com';	//the address where the mail will be send to

	$comment = get_comment($comment_id);

	$from = "From: \"".get_option('blogname')." Guestbook\" <$guestbooksender>";
	$reply_to = "Reply-To: $comment->comment_author_email";
	$subject 		 = sprintf( __('New message in your guestbook'));
	$message_headers = "MIME-Version: 1.0\n". "$from\n"."Content-Type: text/plain; charset=\"" .get_option('blog_charset')."\"\n";
	$notify_message  = sprintf( __('New message in your guestbook')) . "\r\n";
	$notify_message .= sprintf( __('Author : %1$s'), $comment->comment_author) . "\r\n";
	//and here are some more lines added, but to keep it readable in this forum, I deleted them.
	//the lines I deleted are the same as the above line; they add a line to the text that is being mailed

	$notify_message = apply_filters('comment_notification_text', $notify_message, $comment_id);
	$subject = apply_filters('comment_notification_subject', $subject, $comment_id);
	$message_headers = apply_filters('comment_notification_headers', $message_headers, $comment_id);

	@wp_mail($guestbookreceiver, $subject, $notify_message, $message_headers);

	return true;
}
endif;
?>

But when I activate this plugin, it does what I expected it to do; the mail-text is changed and it is being send to the specified mail-address in the plugin, but I’m not able to publish new posts anymore. When I write a new post, and I press the publish-button, WordPress tells me that my attempt to edit the post failed.

Do you know a solution for my problem?
Or do you know another solution to change the text and the recipient of these mails?

Thank you very much!

MaartenN