Update README.md

README.md

@@ -36,6 +36,9 @@ The following env variables need to be passed to the container:
 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.
 
+* `SMTP_NETWORKS` Setting this will allow you to add additional, comma seperated, subnets to use the relay. Used like
+    -e SMTP_NETWORKS='xxx.xxx.xxx.xxx/xx,xxx.xxx.xxx.xxx/xx'
+
 To use this container from anywhere, the 25 port or the one specified by `SMTP_PORT` needs to be exposed to the docker host server:
 
     docker run -d --name postfix -p "25:25"  \