Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -2175,6 +2175,7 @@
     'PhabricatorUserCustomFieldStringIndex' => 'applications/people/storage/PhabricatorUserCustomFieldStringIndex.php',
     'PhabricatorUserDAO' => 'applications/people/storage/PhabricatorUserDAO.php',
     'PhabricatorUserEditor' => 'applications/people/editor/PhabricatorUserEditor.php',
+    'PhabricatorUserEditorTestCase' => 'applications/people/editor/__tests__/PhabricatorUserEditorTestCase.php',
     'PhabricatorUserEmail' => 'applications/people/storage/PhabricatorUserEmail.php',
     'PhabricatorUserLog' => 'applications/people/storage/PhabricatorUserLog.php',
     'PhabricatorUserPreferences' => 'applications/settings/storage/PhabricatorUserPreferences.php',
@@ -4997,6 +4998,7 @@
     'PhabricatorUserCustomFieldStringIndex' => 'PhabricatorCustomFieldStringIndexStorage',
     'PhabricatorUserDAO' => 'PhabricatorLiskDAO',
     'PhabricatorUserEditor' => 'PhabricatorEditor',
+    'PhabricatorUserEditorTestCase' => 'PhabricatorTestCase',
     'PhabricatorUserEmail' => 'PhabricatorUserDAO',
     'PhabricatorUserLog' => 'PhabricatorUserDAO',
     'PhabricatorUserPreferences' => 'PhabricatorUserDAO',
Index: src/applications/auth/controller/PhabricatorAuthRegisterController.php
===================================================================
--- src/applications/auth/controller/PhabricatorAuthRegisterController.php
+++ src/applications/auth/controller/PhabricatorAuthRegisterController.php
@@ -200,8 +200,11 @@
         if (!strlen($value_email)) {
           $e_email = pht('Required');
           $errors[] = pht('Email is required.');
-        } else if (!PhabricatorUserEmail::isAllowedAddress($value_email)) {
+        } else if (!PhabricatorUserEmail::isValidAddress($value_email)) {
           $e_email = pht('Invalid');
+          $errors[] = PhabricatorUserEmail::describeValidAddresses();
+        } else if (!PhabricatorUserEmail::isAllowedAddress($value_email)) {
+          $e_email = pht('Disallowed');
           $errors[] = PhabricatorUserEmail::describeAllowedAddresses();
         } else {
           $e_email = null;
Index: src/applications/people/editor/PhabricatorUserEditor.php
===================================================================
--- src/applications/people/editor/PhabricatorUserEditor.php
+++ src/applications/people/editor/PhabricatorUserEditor.php
@@ -570,6 +570,10 @@
     // user friendly errors for us, but we omit the courtesy checks on some
     // pathways like administrative scripts for simplicity.
 
+    if (!PhabricatorUserEmail::isValidAddress($email->getAddress())) {
+      throw new Exception(PhabricatorUserEmail::describeValidAddresses());
+    }
+
     if (!PhabricatorUserEmail::isAllowedAddress($email->getAddress())) {
       throw new Exception(PhabricatorUserEmail::describeAllowedAddresses());
     }
Index: src/applications/people/editor/__tests__/PhabricatorUserEditorTestCase.php
===================================================================
--- /dev/null
+++ src/applications/people/editor/__tests__/PhabricatorUserEditorTestCase.php
@@ -0,0 +1,68 @@
+<?php
+
+final class PhabricatorUserEditorTestCase extends PhabricatorTestCase {
+
+  protected function getPhabricatorTestCaseConfiguration() {
+    return array(
+      self::PHABRICATOR_TESTCONFIG_BUILD_STORAGE_FIXTURES => true,
+    );
+  }
+
+  public function testRegistrationEmailOK() {
+    $env = PhabricatorEnv::beginScopedEnv();
+    $env->overrideEnvConfig('auth.email-domains', array('example.com'));
+
+    $this->registerUser(
+      'PhabricatorUserEditorTestCaseOK',
+      'PhabricatorUserEditorTestCase@example.com');
+  }
+
+  public function testRegistrationEmailInvalid() {
+    $env = PhabricatorEnv::beginScopedEnv();
+    $env->overrideEnvConfig('auth.email-domains', array('example.com'));
+
+    $prefix = str_repeat('a', PhabricatorUserEmail::MAX_ADDRESS_LENGTH);
+    $email = $prefix.'@evil.com@example.com';
+
+    try {
+      $this->registerUser(
+        'PhabricatorUserEditorTestCaseInvalid',
+        $email);
+    } catch (Exception $ex) {
+      $caught = $ex;
+    }
+
+    $this->assertEqual(true, ($caught instanceof Exception));
+  }
+
+  public function testRegistrationEmailDomain() {
+    $env = PhabricatorEnv::beginScopedEnv();
+    $env->overrideEnvConfig('auth.email-domains', array('example.com'));
+
+    $caught = null;
+    try {
+      $this->registerUser(
+        'PhabricatorUserEditorTestCaseDomain',
+        'PhabricatorUserEditorTestCase@whitehouse.gov');
+    } catch (Exception $ex) {
+      $caught = $ex;
+    }
+
+    $this->assertEqual(true, ($caught instanceof Exception));
+  }
+
+  private function registerUser($username, $email) {
+    $user = id(new PhabricatorUser())
+      ->setUsername($username)
+      ->setRealname($username);
+
+    $email = id(new PhabricatorUserEmail())
+      ->setAddress($email)
+      ->setIsVerified(0);
+
+    id(new PhabricatorUserEditor())
+      ->setActor($user)
+      ->createNewUser($user, $email);
+  }
+
+}
Index: src/applications/people/storage/PhabricatorUserEmail.php
===================================================================
--- src/applications/people/storage/PhabricatorUserEmail.php
+++ src/applications/people/storage/PhabricatorUserEmail.php
@@ -12,6 +12,8 @@
   protected $isPrimary;
   protected $verificationCode;
 
+  const MAX_ADDRESS_LENGTH = 128;
+
   public function getVerificationURI() {
     return '/emailverify/'.$this->getVerificationCode().'/';
   }
@@ -30,7 +32,33 @@
   /**
    * @task restrictions
    */
+  public static function isValidAddress($address) {
+    if (strlen($address) > self::MAX_ADDRESS_LENGTH) {
+      return false;
+    }
+
+    return true;
+  }
+
+
+  /**
+   * @task restrictions
+   */
+  public static function describeValidAddresses() {
+    return pht(
+      'The maximum length of an email address is %d character(s).',
+      new PhutilNumber(self::MAX_ADDRESS_LENGTH));
+  }
+
+
+  /**
+   * @task restrictions
+   */
   public static function isAllowedAddress($address) {
+    if (!self::isValidAddress($address)) {
+      return false;
+    }
+
     $allowed_domains = PhabricatorEnv::getEnvConfig('auth.email-domains');
     if (!$allowed_domains) {
       return true;
Index: src/applications/settings/panel/PhabricatorSettingsPanelEmailAddresses.php
===================================================================
--- src/applications/settings/panel/PhabricatorSettingsPanelEmailAddresses.php
+++ src/applications/settings/panel/PhabricatorSettingsPanelEmailAddresses.php
@@ -173,8 +173,11 @@
       if (!strlen($email)) {
         $e_email = pht('Required');
         $errors[] = pht('Email is required.');
-      } else if (!PhabricatorUserEmail::isAllowedAddress($email)) {
+      } else if (!PhabricatorUserEmail::isValidAddress($email)) {
         $e_email = pht('Invalid');
+        $errors[] = PhabricatorUserEmail::describeValidAddresses();
+      } else if (!PhabricatorUserEmail::isAllowedAddress($email)) {
+        $e_email = pht('Disallowed');
         $errors[] = PhabricatorUserEmail::describeAllowedAddresses();
       }