Updated with instructions.

README.md

@@ -1,2 +1,39 @@
 # docker-postfix
-Simple SMTP relay docker image.
+Simple Postfix SMTP TLS relay docker image.Run it like this:
+
+### Build instructions
+
+Clone this repo and then:
+
+    cd docker-Postfix
+    sudo docker build -t postfix .
+
+You can also find a prebuilt docker image from [Docker Hub](https://registry.hub.docker.com/u/juanluisbaptiste/postfix/), which can be pulled from the Hub:
+
+    sudo docker pull juanluisbaptiste/postfix:latest
+
+### How to run it
+
+The following env variables need to be passed to the container:
+
+* `SMTP_SERVER` Server address of the SMTP server to use.
+* `SMTP_USERNAME` Username to authenticate with.
+* `SMTP_PASSWORD` Password of the SMTP user.
+
+To use this container from anywhere, the port 25 needs to be exposed to the docker host server:
+
+    docker run --rm --name postfix -p "25:25" -e SMTP_SERVER=smtp.bar.com -e SMTP_USERNAME=foo@bar.com -e SMTP_PASSWORD=XXXXXXXX postfix
+    
+If you are going to use this container from other docker containers then it's better to just publish the port:
+
+    docker run --rm --name postfix -P -e SMTP_SERVER=smtp.bar.com -e SMTP_USERNAME=foo@bar.com -e SMTP_PASSWORD=XXXXXXXX postfix
+    
+    
+## A note about using gmail as a relay
+
+Since last year, Gmail by default (does not allow email clients that don't use OAUTH 2)[http://googleonlinesecurity.blogspot.co.uk/2014/04/new-security-measures-will-affect-older.html] 
+for authentication (like Thunderbird or Outlook). First you need to enable access to "Less secure apps" on your 
+[google settings](https://www.google.com/settings/security/lesssecureapps). 
+
+Also take into account that email `From:` header will contain the email address of the account being used to 
+authenticate against the Gmail SMTP server(SMTP_USERNAME), the one on the email will be ignored by Gmail.