diff --git a/Dockerfile b/Dockerfile
index 8b782ce..884a715 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -39,12 +39,12 @@ ENV POSTSCREEN_DNSBL_THRESHOLD="3"
 ENV POSTSCREEN_DNSBL_WHITELIST_THRESHOLD="-1"
 ENV POSTSCREEN_GREET_ACTION="enforce"
 ENV RELAYHOST=""
-ENV SMTP_SASL_AUTH_ENABLE=no
+ENV SMTP_SASL_AUTH_ENABLE=""
 ENV SMTP_SASL_PASSWORD_MAPS=""
 ENV SMTP_SASL_SECURITY_OPTIONS=""
 ENV SMTP_TLS_CAFILE=""
 ENV SMTP_TLS_MANDATORY_PROTOCOLS=""
-ENV SMTP_TLS_NOTE_STARTTLS_OFFER=no
+ENV SMTP_TLS_NOTE_STARTTLS_OFFER=""
 ENV SMTP_TLS_SECURITY_LEVEL=""
 ENV SMTP_TLS_SESSION_CACHE_DATABASE=""