diff --git a/infrastructure/platform/mail/mailu-helm/prod/values.yaml b/infrastructure/platform/mail/mailu-helm/prod/values.yaml
index 6b9bc52b..d9708e9f 100644
--- a/infrastructure/platform/mail/mailu-helm/prod/values.yaml
+++ b/infrastructure/platform/mail/mailu-helm/prod/values.yaml
@@ -42,8 +42,10 @@ initialAccount:
 externalRelay:
   host: "[smtp.mailersend.net]:2525"
   # Credentials loaded from existing Kubernetes secret
-  # Key names use Helm chart defaults (relay-username, relay-password)
   existingSecret: "mailu-mailersend-credentials"
+  # CRITICAL: Explicitly specify key names to ensure Helm uses our secret
+  usernameKey: "relay-username"
+  passwordKey: "relay-password"
 
 # Postfix configuration
 # CRITICAL: podAnnotations ensures Postfix restarts when credentials change
diff --git a/infrastructure/platform/mail/mailu-helm/values.yaml b/infrastructure/platform/mail/mailu-helm/values.yaml
index 67a9b0ab..a26ecf9f 100644
--- a/infrastructure/platform/mail/mailu-helm/values.yaml
+++ b/infrastructure/platform/mail/mailu-helm/values.yaml
@@ -57,8 +57,10 @@ limits:
 externalRelay:
   host: "[smtp.mailersend.net]:587"
   # Use existing secret for credentials (recommended for security)
-  # Key names use Helm chart defaults: relay-username, relay-password
   existingSecret: "mailu-mailersend-credentials"
+  # CRITICAL: Explicitly specify key names to ensure Helm uses our secret
+  usernameKey: "relay-username"
+  passwordKey: "relay-password"
 
 # Webmail configuration
 webmail: