diff --git a/src/applications/config/option/PhabricatorMetaMTAConfigOptions.php b/src/applications/config/option/PhabricatorMetaMTAConfigOptions.php
--- a/src/applications/config/option/PhabricatorMetaMTAConfigOptions.php
+++ b/src/applications/config/option/PhabricatorMetaMTAConfigOptions.php
@@ -191,10 +191,7 @@
       $this->newOption('cluster.mailers', 'cluster.mailers', array())
         ->setHidden(true)
         ->setDescription($mailers_description),
-      $this->newOption(
-        'metamta.default-address',
-        'string',
-        'noreply@phabricator.example.com')
+      $this->newOption('metamta.default-address', 'string', null)
         ->setDescription(pht('Default "From" address.')),
       $this->newOption(
         'metamta.one-mail-per-recipient',
diff --git a/src/applications/metamta/editor/PhabricatorMetaMTAApplicationEmailEditor.php b/src/applications/metamta/editor/PhabricatorMetaMTAApplicationEmailEditor.php
--- a/src/applications/metamta/editor/PhabricatorMetaMTAApplicationEmailEditor.php
+++ b/src/applications/metamta/editor/PhabricatorMetaMTAApplicationEmailEditor.php
@@ -104,6 +104,16 @@
               pht('Invalid'),
               pht('Email address is not formatted properly.'));
           }
+
+          $address = new PhutilEmailAddress($email);
+          if (PhabricatorMailUtil::isReservedAddress($address)) {
+            $errors[] = new PhabricatorApplicationTransactionValidationError(
+              $type,
+              pht('Reserved'),
+              pht(
+                'This email address is reserved. Choose a different '.
+                'address.'));
+          }
         }
 
         $missing = $this->validateIsEmptyTextField(
diff --git a/src/applications/metamta/receiver/__tests__/PhabricatorMailReceiverTestCase.php b/src/applications/metamta/receiver/__tests__/PhabricatorMailReceiverTestCase.php
--- a/src/applications/metamta/receiver/__tests__/PhabricatorMailReceiverTestCase.php
+++ b/src/applications/metamta/receiver/__tests__/PhabricatorMailReceiverTestCase.php
@@ -41,4 +41,30 @@
     }
   }
 
+  public function testReservedAddresses() {
+    $default_address = id(new PhabricatorMetaMTAMail())
+      ->newDefaultEmailAddress();
+
+    $void_address = id(new PhabricatorMetaMTAMail())
+      ->newVoidEmailAddress();
+
+    $map = array(
+      'alincoln@example.com' => false,
+      'sysadmin@example.com' => true,
+      'hostmaster@example.com' => true,
+      '"Walter Ebmaster" <webmaster@example.com>' => true,
+      (string)$default_address => true,
+      (string)$void_address => true,
+    );
+
+    foreach ($map as $raw_address => $expect) {
+      $address = new PhutilEmailAddress($raw_address);
+
+      $this->assertEqual(
+        $expect,
+        PhabricatorMailUtil::isReservedAddress($address),
+        pht('Reserved: %s', $raw_address));
+    }
+  }
+
 }
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
@@ -713,7 +713,7 @@
     $actors = $this->loadAllActors();
     $deliverable_actors = $this->filterDeliverableActors($actors);
 
-    $default_from = PhabricatorEnv::getEnvConfig('metamta.default-address');
+    $default_from = (string)$this->newDefaultEmailAddress();
     if (empty($params['from'])) {
       $mailer->setFrom($default_from);
     }
@@ -1463,18 +1463,33 @@
   }
 
   private function newMailDomain() {
+    $domain = PhabricatorEnv::getEnvConfig('metamta.reply-handler-domain');
+    if (strlen($domain)) {
+      return $domain;
+    }
+
     $install_uri = PhabricatorEnv::getURI('/');
     $install_uri = new PhutilURI($install_uri);
 
     return $install_uri->getDomain();
   }
 
-  public function newVoidEmailAddress() {
+  public function newDefaultEmailAddress() {
+    $raw_address = PhabricatorEnv::getEnvConfig('metamta.default-address');
+    if (strlen($raw_address)) {
+      return new PhutilEmailAddress($raw_address);
+    }
+
     $domain = $this->newMailDomain();
-    $address = "void-recipient@{$domain}";
+    $address = "noreply@{$domain}";
+
     return new PhutilEmailAddress($address);
   }
 
+  public function newVoidEmailAddress() {
+    return $this->newDefaultEmailAddress();
+  }
+
 
 /* -(  Routing  )------------------------------------------------------------ */
 
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAReceivedMail.php b/src/applications/metamta/storage/PhabricatorMetaMTAReceivedMail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAReceivedMail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAReceivedMail.php
@@ -161,10 +161,19 @@
         ->setFilterMethod('isEnabled')
         ->execute();
 
+      $targets = $this->newTargetAddresses();
+      foreach ($targets as $key => $target) {
+        // Never accept any reserved address as a mail target. This prevents
+        // security issues around "hostmaster@" and bad behavior with
+        // "noreply@".
+        if (PhabricatorMailUtil::isReservedAddress($target)) {
+          unset($targets[$key]);
+          continue;
+        }
+      }
+
       $any_accepted = false;
       $receiver_exception = null;
-
-      $targets = $this->newTargetAddresses();
       foreach ($receivers as $receiver) {
         $receiver = id(clone $receiver)
           ->setViewer($viewer);
diff --git a/src/applications/metamta/util/PhabricatorMailUtil.php b/src/applications/metamta/util/PhabricatorMailUtil.php
--- a/src/applications/metamta/util/PhabricatorMailUtil.php
+++ b/src/applications/metamta/util/PhabricatorMailUtil.php
@@ -62,4 +62,50 @@
     return ($u->getAddress() === $v->getAddress());
   }
 
+  public static function isReservedAddress(PhutilEmailAddress $address) {
+    $address = self::normalizeAddress($address);
+    $local = $address->getLocalPart();
+
+    $reserved = array(
+      'admin',
+      'administrator',
+      'hostmaster',
+      'list',
+      'list-request',
+      'majordomo',
+      'postmaster',
+      'root',
+      'ssl-admin',
+      'ssladmin',
+      'ssladministrator',
+      'sslwebmaster',
+      'sysadmin',
+      'uucp',
+      'webmaster',
+
+      'noreply',
+      'no-reply',
+    );
+
+    $reserved = array_fuse($reserved);
+
+    if (isset($reserved[$local])) {
+      return true;
+    }
+
+    $default_address = id(new PhabricatorMetaMTAMail())
+      ->newDefaultEmailAddress();
+    if (self::matchAddresses($address, $default_address)) {
+      return true;
+    }
+
+    $void_address = id(new PhabricatorMetaMTAMail())
+      ->newVoidEmailAddress();
+    if (self::matchAddresses($address, $void_address)) {
+      return true;
+    }
+
+    return false;
+  }
+
 }