diff --git a/Dockerfile b/Dockerfile
index 2af82e0..f6ce004 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -40,6 +40,7 @@ ENV POSTSCREEN_DNSBL_THRESHOLD="3"
 ENV POSTSCREEN_DNSBL_WHITELIST_THRESHOLD="-1"
 ENV POSTSCREEN_GREET_ACTION="enforce"
 ENV RELAYHOST=""
+ENV SMTP_FALLBACK_RELAY=""
 ENV SMTP_SASL_AUTH_ENABLE=""
 ENV SMTP_SASL_PASSWORD_MAPS=""
 ENV SMTP_SASL_SECURITY_OPTIONS=""