First, find the module file:

modules/contact/contact.module

If you prefer command line editing open up that file in your favorite editor. If you prefer the download/edit/upload approach via FTP that will work also. I assume if you have interest in this article, that you have some knowledge of either of those 2 methods. If not, I wonder how you set up Drupal in the first place.

Now that you are in edit mode, find a chunk of code that talks about not allowing anonymous users to send themselves a copy for spam reasons. Found that comment? Good, now just above it you are going to add your field. I found it easy to copy/paste an existing form element and edit it’s contents, but you can type it out if you like. Actually, feel free to copy/paste and edit my example below. Here’s what I ended up with:

 $form['newsletter'] = array('#type' => 'checkbox',
    '#title' => t('Sign me up for the Newsletter'),
  );

You may want to add a comment somewhere in there to remind yourself at a later date that this is no longer part of the original configuration.

That was easy, now I just need to tell it to add content to the email sent out so that when that box is checked, it actually does something! Next look for a chunk of comment code that looks like this:

// Compose the body:

below that you will find a couple lines that start with “$message[] =” which is where the body of the email’s message gets created. Just add another line that says something like the following:

 if ($form_values['newsletter'] == 1) {
    $message[] = "Yes, please send me a newsletter";
  } else {
    $message[] = "No, please do not send me a newsletter";
  }

Now crank open your form and test it out. Assuming you are the recipient of the email being generated, you will get a nice extra piece of text at the bottom of the email with this new content based on the user’s input.