diff --git a/run.sh b/run.sh
index 46763cb..49daade 100644
--- a/run.sh
+++ b/run.sh
@@ -47,7 +47,7 @@ if [ ! -f /etc/postfix/sasl_passwd ]; then
 fi
 
 #Set header tag  
-if [ -z "${SMTP_HEADER_TAG}" ]; then
+if [ ! -z "${SMTP_HEADER_TAG}" ]; then
   postconf -e "header_checks = regexp:/etc/postfix/header_tag"
   echo -e "/^MIME-Version:/i PREPEND RelayTag: $SMTP_HEADER_TAG\n/^Content-Transfer-Encoding:/i PREPEND RelayTag: $SMTP_HEADER_TAG" > /etc/postfix/header_tag
   echo "Setting configuration option SMTP_HEADER_TAG with value: ${SMTP_HEADER_TAG}"