diff --git a/compose.yaml b/compose.yaml
index 1dbdd93..f429a69 100644
--- a/compose.yaml
+++ b/compose.yaml
@@ -1,6 +1,10 @@
 services:
   mailserver:
-    image: docker.io/mailserver/docker-mailserver:latest
+    # Pinned to v15.1.0: the config (dovecot.cf nameless shared namespace,
+    # dovecot-quotas.cf) targets the Dovecot 2.3 of v15. docker.io:latest now
+    # pulls v16 (Dovecot 2.4) which rejects the config — deliberate upgrade to
+    # v16 is a separate task (migrate dovecot.cf first).
+    image: docker.io/mailserver/docker-mailserver:v15.1.0
     container_name: mailserver
     hostname: mail.produktor.io
     ports: