diff --git a/reroute_email.admin.inc b/reroute_email.admin.inc
index d755d27..d834211 100644
--- a/reroute_email.admin.inc
+++ b/reroute_email.admin.inc
@@ -15,15 +15,43 @@ function reroute_email_settings() {
     '#default_value' => variable_get(REROUTE_EMAIL_ENABLE, 0),
     '#description'   => t('Check this box if you want to enable email rerouting. Uncheck to disable rerouting.'),
   );
-  $form[REROUTE_EMAIL_ADDRESS] = array(
+  $form[REROUTE_EMAIL_TO] = array(
+    '#type'          => 'textarea',
+    '#title'         => t('Destination email addresses'),
+    '#default_value' => reroute_email_destination(),
+    '#description'   => t('Provide a space, comma, semicolon, or newline-delimited list of email addresses to reroute email messages to.'),
+    '#states' => array(
+      'visible' => array(':input[name=reroute_email_enable]' => array('checked' => TRUE)),
+    ),
+  );
+
+  $form['whitelist'] = array(
+    '#type' => 'fieldset',
+    '#title' => t('Whitelists'),
+    '#description' => t('Permit delivery to these addresses and/or domains. Mail for non-whitelisted email addresses will be rerouted to the destination addresses above.'),
+    '#states' => array(
+      'visible' => array(':input[name=reroute_email_enable]' => array('checked' => TRUE)),
+    ),
+  );
+  $form['whitelist'][REROUTE_EMAIL_WHITELIST] = array(
+    '#type'          => 'textarea',
+    '#title'         => t('Email address whitelist'),
+    '#default_value' => variable_get(REROUTE_EMAIL_WHITELIST, REROUTE_EMAIL_DEFAULT_ADDRESS),
+    '#description'   => t('Provide a space, comma, semicolon, or newline-delimited list of email addresses to permit delivery to.'),
+    '#states' => array(
+      'visible' => array(':input[name=reroute_email_enable]' => array('checked' => TRUE)),
+    ),
+  );
+  $form['whitelist'][REROUTE_EMAIL_DOMAIN_WHITELIST] = array(
     '#type'          => 'textfield',
-    '#title'         => t('Email addresses'),
-    '#default_value' => variable_get(REROUTE_EMAIL_ADDRESS, ini_get('sendmail_from')),
-    '#description'   => t('Provide a space, comma, or semicolon-delimited list of email addresses to pass through. Every destination email address which is not on this list will be rerouted to the first address on the list.'),
+    '#title'         => t('Domain whitelist'),
+    '#default_value' => variable_get(REROUTE_EMAIL_DOMAIN_WHITELIST, ''),
+    '#description'   => t('Provide a space, comma, or semicolon-delimited list of domains to permit delivery to.'),
     '#states' => array(
       'visible' => array(':input[name=reroute_email_enable]' => array('checked' => TRUE)),
     ),
   );
+
   $form[REROUTE_EMAIL_ENABLE_MESSAGE] = array(
     '#type' => 'checkbox',
     '#title' => t('Show rerouting description in mail body'),
@@ -42,11 +70,25 @@ function reroute_email_settings() {
  */
 function reroute_email_settings_validate($form, $form_state) {
   if ($form_state['values']['reroute_email_enable'] == TRUE) {
-    // Allow splitting emails by space, comma, semicolon.
-    $addresslist = preg_split(EMAIL_SPLIT_RE, $form_state['values']['reroute_email_address'], -1, PREG_SPLIT_NO_EMPTY);
-    foreach ($addresslist as $address) {
+    // Destination addresses
+    $destination = reroute_email_split_string($form_state['values'][REROUTE_EMAIL_TO]);
+    foreach ($destination as $address) {
       if (!valid_email_address($address)) {
-        form_set_error('reroute_email_address', t('@address is not a valid email address', array('@address' => $address)));
+        form_set_error(REROUTE_EMAIL_TO, t('@address is not a valid email address', array('@address' => $address)));
+      }
+    }
+    // Whitelisted addresses
+    $whitelist = reroute_email_split_string($form_state['values'][REROUTE_EMAIL_WHITELIST]);
+    foreach ($whitelist as $address) {
+      if (!valid_email_address($address)) {
+        form_set_error(REROUTE_EMAIL_WHITELIST, t('@address is not a valid email address', array('@address' => $address)));
+      }
+    }
+    // Whitelisted domains
+    $domain_whitelist = reroute_email_split_string($form_state['values'][REROUTE_EMAIL_DOMAIN_WHITELIST]);
+    foreach ($domain_whitelist as $domain) {
+      if (!valid_email_address("user@" . $domain)) {
+        form_set_error(REROUTE_EMAIL_DOMAIN_WHITELIST, t('@domain is not a valid domain', array('@domain' => $domain)));
       }
     }
   }
diff --git a/reroute_email.install b/reroute_email.install
index 093a0bf..6cb880d 100644
--- a/reroute_email.install
+++ b/reroute_email.install
@@ -9,7 +9,9 @@
  * Implements hook_uninstall().
  */
 function reroute_email_uninstall() {
+  variable_del('reroute_email_to');
   variable_del('reroute_email_address');
-  variable_del('reroute_email_enable');
+  variable_del('reroute_email_domain_whitelist');
   variable_del('reroute_email_enable_message');
+  variable_del('reroute_email_enable');
 }
diff --git a/reroute_email.module b/reroute_email.module
index df35e17..2116a87 100644
--- a/reroute_email.module
+++ b/reroute_email.module
@@ -5,15 +5,12 @@
  * Reroute Email module
  */
 
-define('REROUTE_EMAIL_ADDRESS', 'reroute_email_address');
+define('REROUTE_EMAIL_TO', 'reroute_email_to');
+define('REROUTE_EMAIL_WHITELIST', 'reroute_email_address'); // historical reasons
+define('REROUTE_EMAIL_DOMAIN_WHITELIST', 'reroute_email_domain_whitelist');
 define('REROUTE_EMAIL_ENABLE_MESSAGE', 'reroute_email_enable_message');
 define('REROUTE_EMAIL_ENABLE', 'reroute_email_enable');
-
-// Regular expression used to split email addresses provided in form.
-// This allows the use of any number of spaces, commas, or semicolons
-// between email addresses.
-define('EMAIL_SPLIT_RE', '/[\s,;]+/');
-
+define('REROUTE_EMAIL_DEFAULT_ADDRESS', ini_get('sendmail_from'));
 
 /**
  * Implements of hook_permission().
@@ -62,6 +59,28 @@ function reroute_email_menu() {
 }
 
 /**
+ * Split string of email addresses (or domains) into an array.
+ *
+ * Items may be separated by any number and combination of:
+ * spaces, commas, semicolons, or newlines.
+ */
+function reroute_email_split_string($string) {
+  return preg_split('/[\s,;\n]+/', $string, -1, PREG_SPLIT_NO_EMPTY);
+}
+
+/**
+ * Return the destination address to reroute email to.
+ *
+ * Historically, the (sole) destination address was simply the
+ * first email address in the whitelist.
+ */
+function reroute_email_destination() {
+  $whitelist = reroute_email_split_string(variable_get(REROUTE_EMAIL_WHITELIST, REROUTE_EMAIL_DEFAULT_ADDRESS));
+  $default_address = array_shift($whitelist);
+  return variable_get(REROUTE_EMAIL_TO, $default_address);
+}
+
+/**
  * Implements hook_mail_alter().
  *
  * This hook is required to change the destination of outgoing emails.
@@ -70,8 +89,9 @@ function reroute_email_mail_alter(&$message) {
   if (variable_get(REROUTE_EMAIL_ENABLE, 0)) {
     global $base_url;
 
-    if (!variable_get(REROUTE_EMAIL_ADDRESS, '')) {
-      // If email address not in settings, then do nothing.
+    $destination = reroute_email_destination();
+    if (!$destination) {
+      // If we don't have a reroute destination, do nothing.
       return;
     }
 
@@ -113,13 +133,21 @@ function reroute_email_mail_alter(&$message) {
       }
     }
 
-    // Split the address string on whitespace, ignoring any empty results.
-    $addresslist = preg_split(EMAIL_SPLIT_RE, variable_get(REROUTE_EMAIL_ADDRESS, ini_get('sendmail_from')), -1, PREG_SPLIT_NO_EMPTY);
+    // Check to see if the To address is whitelisted.
+    // (It is currently assumed that there is only a single address in the To header.)
+    $reroute_required = FALSE;
+    $address_whitelist = reroute_email_split_string(variable_get(REROUTE_EMAIL_WHITELIST, REROUTE_EMAIL_DEFAULT_ADDRESS));
+    if (!in_array($to, $address_whitelist, TRUE)) {
+      $to_domain = array_pop(explode('@', $to));
+      $domain_whitelist = reroute_email_split_string(variable_get(REROUTE_EMAIL_DOMAIN_WHITELIST, ''));
+      if (!in_array($to_domain, $domain_whitelist, TRUE)) {
+        $reroute_required = TRUE;
+      }
+    }
 
-    if (!in_array($to, $addresslist)) {
-      // Not on the list, so reroute to the first address in the list.
+    if ($reroute_required) {
       $message['headers']['X-Rerouted-Original-To'] = $to;
-      $message['to'] = $addresslist[0];
+      $message['to'] = implode(',', reroute_email_split_string($destination));
 
       if (variable_get(REROUTE_EMAIL_ENABLE_MESSAGE, 1)) {
         // Format a message to show at the top.
diff --git a/reroute_email.test b/reroute_email.test
index 5a6a52a..284af07 100644
--- a/reroute_email.test
+++ b/reroute_email.test
@@ -46,7 +46,7 @@ class RerouteEmailTestCase extends DrupalWebTestCase {
 
     // Configure to reroute normally to rerouted@example.com.
     $post = array(
-      'reroute_email_address' => $reroute_destination,
+      'reroute_email_to' => $reroute_destination,
       'reroute_email_enable' => TRUE,
       'reroute_email_enable_message' => TRUE,
     );