Set from e-mail address for new user registration only?
You can use hook_mail_alter() to change the "from" email address of any email sent out from your site. Your code would look something like...

function yourmodule_mail_alter(&amp;$message) {
  $message['from'] = $message['headers']['From'] = '"Full Name" &lt;noreply@mysite.com&gt;';
}


Also, keep in mind that just setting the "from" address may not have the desired results in all email clients (see the discussion on the Drupal issue queue). You might also need to set "sender", "errors-to" and "reply-to" based on your requirements. Those values are in the "header" of the $message variable from the code sample above.