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
@@ -103,6 +103,7 @@
               $type,
               pht('Invalid'),
               pht('Email address is not formatted properly.'));
+            continue;
           }
 
           $address = new PhutilEmailAddress($email);
@@ -113,6 +114,19 @@
               pht(
                 'This email address is reserved. Choose a different '.
                 'address.'));
+            continue;
+          }
+
+          // See T13234. Prevent use of user email addresses as application
+          // email addresses.
+          if (PhabricatorMailUtil::isUserAddress($address)) {
+            $errors[] = new PhabricatorApplicationTransactionValidationError(
+              $type,
+              pht('In Use'),
+              pht(
+                'This email address is already in use by a user. Choose '.
+                'a different address.'));
+            continue;
           }
         }
 
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
@@ -170,6 +170,13 @@
           unset($targets[$key]);
           continue;
         }
+
+        // See T13234. Don't process mail if a user has attached this address
+        // to their account.
+        if (PhabricatorMailUtil::isUserAddress($target)) {
+          unset($targets[$key]);
+          continue;
+        }
       }
 
       $any_accepted = false;
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
@@ -108,4 +108,12 @@
     return false;
   }
 
+  public static function isUserAddress(PhutilEmailAddress $address) {
+    $user_email = id(new PhabricatorUserEmail())->loadOneWhere(
+      'address = %s',
+      $address->getAddress());
+
+    return (bool)$user_email;
+  }
+
 }