diff --git a/functions.php b/functions.php
index 6340e53..d162d1f 100644
--- a/functions.php
+++ b/functions.php
@@ -431,3 +431,20 @@ function restrict_rest_api_to_localhost() {
     }
 }
 add_action( 'rest_api_init', 'restrict_rest_api_to_localhost', 0 );
+
+/* Security: filter email domains frequently used for spam registrations */
+function user_registration_filter($user_id, $email) {
+   if ($user_id == false) {
+      $e1 = explode("@", $email);
+      if (in_array($e1[1], array(
+         'xtra.ltd','tmobile.agency','btee.online','verizonconnect.online',
+         'eduonline.digital','vofadonepro.online'))) {
+         error_log( "user_registration_filter: email=" . $email . " blacklisted domain=" . $e1[1] );
+         $user_id = 100;
+      } else {
+         error_log( "user_registration_filter: email=" . $email . " whitelisted domain=" . $e1[1] );
+      }
+   }
+   return $user_id;
+}
+add_filter('email_exists', 'user_registration_filter', 10, 2);