diff --git a/Dockerfile b/Dockerfile
index 9b00401..9b5dcc6 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -11,6 +11,9 @@ ENV SMARTHOST_USER=""
 ENV SMARTHOST_PASS=""
 # ip addresses or networks to allow for relaying, separate multiple ones by semicolon
 ENV RELAYNETS="127.0.0.1/32"
+# whitelist of recipient domains, colon-separated, if empty all no restrictions
+ENV WHITELISTED_RECIPIENTS=""
+
 
 RUN apk add --no-cache exim m4