diff --git a/README.md b/README.md
index d6ca131..5e0ae1d 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,9 @@ The following env variables need to be passed to the container:
 * `SMTP_PASSWORD` Password of the SMTP user.
 * `SERVER_HOSTNAME` Server hostname for the Postfix container. Emails will appear to come from the hostname's domain.
 
+The following env variable(s) are optional.
+* `SMTP_HEADER_TAG` This will add a header for tracking messages upstream. Helpful for spam filters. Will appear as "RelayTag: ${SMTP_HEADER_TAG}" in the email headers.
+
 To use this container from anywhere, the 25 port needs to be exposed to the docker host server:
 
     docker run -d --name postfix -p "25:25"  \ 
diff --git a/run.sh b/run.sh
index aa1ff7d..49daade 100644
--- a/run.sh
+++ b/run.sh
@@ -46,5 +46,12 @@ if [ ! -f /etc/postfix/sasl_passwd ]; then
   fi
 fi
 
+#Set header tag  
+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}"
+fi
+
 #Start services
 supervisord