diff --git a/src/applications/auth/controller/PhabricatorAuthRegisterController.php b/src/applications/auth/controller/PhabricatorAuthRegisterController.php
--- a/src/applications/auth/controller/PhabricatorAuthRegisterController.php
+++ b/src/applications/auth/controller/PhabricatorAuthRegisterController.php
@@ -62,6 +62,16 @@
     if (!PhabricatorUserEmail::isValidAddress($default_email)) {
       $default_email = null;
     }
+    if ($default_email !== null) {
+      // We shoud bypass policy here becase e.g. limiting an application use
+      // to a subset of users should not allow the others to overwrite
+      // configured application emails
+      $application_email = id(new PhabricatorMetaMTAApplicationEmail())
+        ->loadOneWhere('address = %s', $default_email);
+      if ($application_email) {
+        $default_email = null;
+      }
+    }
 
     if ($default_email !== null) {
       // If the account source provided an email, but it's not allowed by
@@ -86,7 +96,6 @@
       // If the account source provided an email, but another account already
       // has that email, just pretend we didn't get an email.
 
-      // TODO: See T3340.
       // TODO: See T3472.
 
       if ($default_email !== null) {
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAApplicationEmail.php b/src/applications/metamta/storage/PhabricatorMetaMTAApplicationEmail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAApplicationEmail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAApplicationEmail.php
@@ -61,6 +61,27 @@
     return idx($this->configData, $key, $default);
   }
 
+
+  public function getInUseMessage() {
+    $applications = PhabricatorApplication::getAllApplications();
+    $applications = mpull($applications, null, 'getPHID');
+    $application = idx(
+      $applications,
+      $this->getApplicationPHID());
+    if ($application) {
+      $message = pht(
+        'The address %s is configured to be used by the %s Application.',
+        $this->getAddress(),
+        $application->getName());
+    } else {
+      $message = pht(
+        'The address %s is configured to be used by an application.',
+        $this->getAddress());
+    }
+
+    return $message;
+  }
+
 /* -(  PhabricatorPolicyInterface  )----------------------------------------- */
 
 
diff --git a/src/applications/people/editor/PhabricatorUserEditor.php b/src/applications/people/editor/PhabricatorUserEditor.php
--- a/src/applications/people/editor/PhabricatorUserEditor.php
+++ b/src/applications/people/editor/PhabricatorUserEditor.php
@@ -578,6 +578,12 @@
     if (!PhabricatorUserEmail::isAllowedAddress($email->getAddress())) {
       throw new Exception(PhabricatorUserEmail::describeAllowedAddresses());
     }
+
+    $application_email = id(new PhabricatorMetaMTAApplicationEmail())
+      ->loadOneWhere('address = %s', $email->getAddress());
+    if ($application_email) {
+      throw new Exception($application_email->getInUseMessage());
+    }
   }
 
   private function revokePasswordResetLinks(PhabricatorUser $user) {
diff --git a/src/applications/people/editor/__tests__/PhabricatorUserEditorTestCase.php b/src/applications/people/editor/__tests__/PhabricatorUserEditorTestCase.php
--- a/src/applications/people/editor/__tests__/PhabricatorUserEditorTestCase.php
+++ b/src/applications/people/editor/__tests__/PhabricatorUserEditorTestCase.php
@@ -53,6 +53,26 @@
     $this->assertTrue($caught instanceof Exception);
   }
 
+  public function testRegistrationEmailApplicationEmailCollide() {
+    $app_email = 'bugs@whitehouse.gov';
+    $app_email_object =
+      PhabricatorMetaMTAApplicationEmail::initializeNewAppEmail(
+        $this->generateNewTestUser());
+    $app_email_object->setAddress($app_email);
+    $app_email_object->setApplicationPHID('test');
+    $app_email_object->save();
+
+    $caught = null;
+    try {
+      $this->registerUser(
+        'PhabricatorUserEditorTestCaseDomain',
+        $app_email);
+    } catch (Exception $ex) {
+      $caught = $ex;
+    }
+    $this->assertTrue($caught instanceof Exception);
+  }
+
   private function registerUser($username, $email) {
     $user = id(new PhabricatorUser())
       ->setUsername($username)
diff --git a/src/applications/settings/panel/PhabricatorEmailAddressesSettingsPanel.php b/src/applications/settings/panel/PhabricatorEmailAddressesSettingsPanel.php
--- a/src/applications/settings/panel/PhabricatorEmailAddressesSettingsPanel.php
+++ b/src/applications/settings/panel/PhabricatorEmailAddressesSettingsPanel.php
@@ -185,6 +185,14 @@
         $e_email = pht('Disallowed');
         $errors[] = PhabricatorUserEmail::describeAllowedAddresses();
       }
+      if ($e_email === true) {
+        $application_email = id(new PhabricatorMetaMTAApplicationEmail())
+          ->loadOneWhere('address = %s', $email);
+        if ($application_email) {
+          $e_email = pht('In Use');
+          $errors[] = $application_email->getInUseMessage();
+        }
+      }
 
       if (!$errors) {
         $object = id(new PhabricatorUserEmail())