diff --git a/README.md b/README.md
index 7027395c27ca5a9c062617ecfc9525a01a393573..30016037af37b52b6cb09005af9e7b6b67c06692 100644
--- a/README.md
+++ b/README.md
@@ -189,6 +189,8 @@ The module knows about the following options:
 * `shard=`*ID* sets the *shard* parameter, which may be used by the
   authentication server to limit authentication to a subset of users.
 
+The `service` parameter sent to the auth server will be the PAM service,
+it is not possible to override this in the PAM configuration.
 
 ## NGINX mail_http_auth support