diff --git a/SECURITY.md b/SECURITY.md
index 8cf18606..cedcd3b0 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -2,7 +2,7 @@
 
 Please disclose any security issues or vulnerabilities found through [Tidelift's coordinated disclosure system](https://tidelift.com/security) or to the maintainers privately.
 
-PHPMailer 6.4.1 and earlier contain a vulnerability that can result in untrusted code being called (if such code is injected into the host project's scope by other means). If the `$patternselect` parameter to `validateAddress()` is set to `'php'` (the default, defined by `static::$validator`), and the global namespace contains a function called `php`, it will be called in preference to the built-in validator of the same name. This is patched in PHPMailer 6.5.0 by denying the use of callables with the same names as built-in validators. Reported by [Vikrant Singh Chauhan](mailto:vi@hackberry.xyz) via [huntr.dev](https://www.huntr.dev/). Recorded as [CVE-2021-3603](https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-3603).
+PHPMailer 6.4.1 and earlier contain a vulnerability that can result in untrusted code being called (if such code is injected into the host project's scope by other means). If the `$patternselect` parameter to `validateAddress()` is set to `'php'` (the default, defined by `static::$validator`), and the global namespace contains a function called `php`, it will be called in preference to the built-in validator of the same name. This is patched in PHPMailer 6.5.0 by denying the use of simple strings as validator function names, which is a very minor BC break. Reported by [Vikrant Singh Chauhan](mailto:vi@hackberry.xyz) via [huntr.dev](https://www.huntr.dev/). Recorded as [CVE-2021-3603](https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-3603).
 
 PHPMailer versions between 6.1.8 and 6.4.0 contain a regression of the earlier CVE-2018-19296 object injection vulnerability as a result of [a fix for Windows UNC paths in 6.1.8](https://github.com/PHPMailer/PHPMailer/commit/e2e07a355ee8ff36aba21d0242c5950c56e4c6f9). Recorded as [CVE-2020-36326](https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2020-36326). Reported by Fariskhi Vidyan via Tidelift. 6.4.1 fixes this issue, and also enforces stricter checks for URL schemes in local path contexts.
 
diff --git a/src/PHPMailer.php b/src/PHPMailer.php
index 83733641..032ef1cf 100644
--- a/src/PHPMailer.php
+++ b/src/PHPMailer.php
@@ -1337,12 +1337,8 @@ class PHPMailer
         if (null === $patternselect) {
             $patternselect = static::$validator;
         }
-        //Don't allow overriding built-in validators with callables
-        if (
-            is_callable($patternselect) &&
-            //It's callable and not a string, or it's a string callable that's not a built-in pattern
-            (!is_string($patternselect) || !in_array(strtolower($patternselect), ['php', 'pcre', 'pcre8', 'html5']))
-        ) {
+        //Don't allow strings as callables, see SECURITY.md and CVE-2021-3603
+        if (is_callable($patternselect) && !is_string($patternselect)) {
             return call_user_func($patternselect, $address);
         }
         //Reject line breaks in addresses; it's valid RFC5322, but not RFC5321
diff --git a/test/PHPMailerTest.php b/test/PHPMailerTest.php
index b6c13f06..e6aa4c77 100644
--- a/test/PHPMailerTest.php
+++ b/test/PHPMailerTest.php
@@ -733,13 +733,14 @@ final class PHPMailerTest extends TestCase
             'PHP validator not behaving as expected'
         );
 
-        //Test denying override of built-in validator names
+        //Test denying function name callables as validators
         //See SECURITY.md and CVE-2021-3603
         //If a `php` function defined in validators.php successfully overrides this built-in validator name,
         //this would return false – and we don't want to allow that
         self::assertTrue(PHPMailer::validateAddress('test@example.com', 'php'));
-        //Check a non-matching validator function, which should be permitted, and return false in this case
-        self::assertFalse(PHPMailer::validateAddress('test@example.com', 'phpx'));
+        //Check that a non-existent validator name falls back to a built-in validator
+        //and does not call a global function with that name
+        self::assertTrue(PHPMailer::validateAddress('test@example.com', 'phpx'));
     }
 
     /**