Change the email sender from a new post

Dear Carbonize/Auron,

I would like to change the email sender when a new post is made. I filled in the webmaster e-mail in the admin panel and enabled the options:

- send e-mail to webmaster (private messages)
- send e-mail to webmaster

Now is the situation that I receive an email when a new post is made, but the sender from the email is or the email address the poster filled in or the ip-address from the poster. I would like to change this to an email address I want, for example:

newmessage@guestbook.nl

In which file can I edit this information? I have been searching the forum but haven't found anything.

Best regards,

Nick Noordenbos