Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.632
diff -u -p -r1.632 common.inc
--- includes/common.inc	18 Apr 2007 20:35:45 -0000	1.632
+++ includes/common.inc	23 Apr 2007 21:31:05 -0000
@@ -1969,8 +1969,9 @@ function drupal_mail($mailkey, $to, $sub
     'Content-Transfer-Encoding' => '8Bit',
     'X-Mailer' => 'Drupal'
   );
-  if (isset($from)) {
-    $defaults['From'] = $defaults['Reply-To'] = $defaults['Return-Path'] = $defaults['Errors-To'] = $from;
+  $default_from = isset($from) ? $from : variable_get('site_mail', ini_get('sendmail_from'));
+  if ($default_from) {
+    $defaults['From'] = $defaults['Reply-To'] = $defaults['Return-Path'] = $defaults['Errors-To'] = $default_from;
   }
   $headers = array_merge($defaults, $headers);
 
Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.470
diff -u -p -r1.470 system.module
--- modules/system/system.module	20 Apr 2007 08:44:01 -0000	1.470
+++ modules/system/system.module	23 Apr 2007 21:31:06 -0000
@@ -574,7 +574,8 @@ function system_site_information_setting
     '#type' => 'textfield',
     '#title' => t('E-mail address'),
     '#default_value' => variable_get('site_mail', ini_get('sendmail_from')),
-    '#description' => t('A valid e-mail address for this website, used by the auto-mailer during registration, new password requests, notifications, etc.')
+    '#description' => t('A valid e-mail address for this website, used as the From address by the auto-mailer during registration, new password requests, notifications, etc.'),
+    '#required' => TRUE,
   );
   $form['site_slogan'] = array(
     '#type' => 'textfield',