diff --git a/Dockerfile b/Dockerfile
index 874bd61..33dacbd 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -24,7 +24,7 @@ RUN apk --no-cache \
 # set environment variables
 ENV TZ=Etc/UTC
 ENV LOCAL_HOSTNAME=${HOSTNAME}
-ENV LOCAL_DOMAINNAME=${HOSTNAME#*.}
+ENV LOCAL_DOMAINNAME=""
 ENV LOCAL_ENCRYPTION=false
 ENV SMARTHOST=""
 ENV SMARTHOST_PORT=587
diff --git a/entrypoint.sh b/entrypoint.sh
index eab1950..475193c 100755
--- a/entrypoint.sh
+++ b/entrypoint.sh
@@ -24,7 +24,7 @@ fi
 # set failsafes
 [ -z "$SMARTHOST_PORT" ] && SMARTHOST_PORT=587
 [ -z "$LOCAL_HOSTNAME" ] && LOCAL_HOSTNAME="smarthost"
-[ -z "$LOCAL_DOMAINNAME" ] && LOCAL_DOMAINNAME="smarthost"
+[ -z "$LOCAL_DOMAINNAME" ] && LOCAL_DOMAINNAME="${LOCAL_HOSTNAME#*.}"
 
 printf "done\n"
 printf "updating configuration files... "