diff --git a/helfi_api_base.module b/helfi_api_base.module
index 943fbcbd..b14d5494 100644
--- a/helfi_api_base.module
+++ b/helfi_api_base.module
@@ -50,3 +50,18 @@ function helfi_api_base_theme_suggestions_debug_item(array $variables) : array {
   $suggestions[] = 'debug_item__' . strtr($variables['id'], '.', '_');
   return $suggestions;
 }
+
+/**
+ * Implements hook_mail_alter().
+ */
+function helfi_api_base_mail_alter(&$message) : void {
+  // Prevent sending email if current site/environment is known to the resolver.
+  // Only helfi-drupal sites are affected by this change.
+  try {
+    \Drupal::service('helfi_api_base.environment_resolver')
+      ->getActiveEnvironment();
+    $message['send'] = FALSE;
+  }
+  catch (\InvalidArgumentException) {
+  }
+}