Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -2177,6 +2177,7 @@
     'PhabricatorUserEditor' => 'applications/people/editor/PhabricatorUserEditor.php',
     'PhabricatorUserEditorTestCase' => 'applications/people/editor/__tests__/PhabricatorUserEditorTestCase.php',
     'PhabricatorUserEmail' => 'applications/people/storage/PhabricatorUserEmail.php',
+    'PhabricatorUserEmailTestCase' => 'applications/people/storage/__tests__/PhabricatorUserEmailTestCase.php',
     'PhabricatorUserLog' => 'applications/people/storage/PhabricatorUserLog.php',
     'PhabricatorUserPreferences' => 'applications/settings/storage/PhabricatorUserPreferences.php',
     'PhabricatorUserProfile' => 'applications/people/storage/PhabricatorUserProfile.php',
@@ -5000,6 +5001,7 @@
     'PhabricatorUserEditor' => 'PhabricatorEditor',
     'PhabricatorUserEditorTestCase' => 'PhabricatorTestCase',
     'PhabricatorUserEmail' => 'PhabricatorUserDAO',
+    'PhabricatorUserEmailTestCase' => 'PhabricatorTestCase',
     'PhabricatorUserLog' => 'PhabricatorUserDAO',
     'PhabricatorUserPreferences' => 'PhabricatorUserDAO',
     'PhabricatorUserProfile' => 'PhabricatorUserDAO',
Index: src/applications/people/storage/PhabricatorUserEmail.php
===================================================================
--- src/applications/people/storage/PhabricatorUserEmail.php
+++ src/applications/people/storage/PhabricatorUserEmail.php
@@ -37,6 +37,23 @@
       return false;
     }
 
+    // Very roughly validate that this address isn't so mangled that a
+    // reasonable piece of code might completely misparse it. In particular,
+    // the major risks are:
+    //
+    //   - `PhutilEmailAddress` needs to be able to extract the domain portion
+    //     from it.
+    //   - Reasonable mail adapters should be hard-pressed to interpret one
+    //     address as several addresses.
+    //
+    // To this end, we're roughly verifying that there's some normal text, an
+    // "@" symbol, and then some more normal text.
+
+    $email_regex = '(^[a-z0-9_+.!-]+@[a-z0-9_+:.-]+$)i';
+    if (!preg_match($email_regex, $address)) {
+      return false;
+    }
+
     return true;
   }
 
@@ -46,7 +63,8 @@
    */
   public static function describeValidAddresses() {
     return pht(
-      'The maximum length of an email address is %d character(s).',
+      "Email addresses should be in the form 'user@domain.com'. The maximum ".
+      "length of an email address is %d character(s).",
       new PhutilNumber(self::MAX_ADDRESS_LENGTH));
   }
 
Index: src/applications/people/storage/__tests__/PhabricatorUserEmailTestCase.php
===================================================================
--- /dev/null
+++ src/applications/people/storage/__tests__/PhabricatorUserEmailTestCase.php
@@ -0,0 +1,41 @@
+<?php
+
+final class PhabricatorUserEmailTestCase extends PhabricatorTestCase {
+
+  public function testEmailValidation() {
+    $tests = array(
+      'alincoln@whitehouse.gov' => true,
+      '_-.@.-_' => true,
+      '.@.com' => true,
+      'user+suffix@gmail.com' => true,
+      'IAMIMPORTANT@BUSINESS.COM' => true,
+      '1@22.33.44.55' => true,
+      '999@999.999' => true,
+      'user@2001:0db8:85a3:0042:1000:8a2e:0370:7334' => true,
+      '!..!@o.O' => true,
+
+      '' => false,
+      str_repeat('a', 256).'@example.com' => false,
+      'quack' => false,
+      '@gmail.com' => false,
+      'usergmail.com' => false,
+      '"user" user@gmail.com' => false,
+      'a,b@evil.com' => false,
+      'a;b@evil.com' => false,
+      'ab@evil.com;cd@evil.com' => false,
+      'x@y@z.com' => false,
+      '@@' => false,
+      '@' => false,
+      'user@' => false,
+    );
+
+    foreach ($tests as $input => $expect) {
+      $actual = PhabricatorUserEmail::isValidAddress($input);
+      $this->assertEqual(
+        $expect,
+        $actual,
+        $input);
+    }
+  }
+
+}