When you hit the Supermail button you'll see:
In the Compose tab you can change such information:
Your name - by default it will be your WebNative username. You can change it in Supermail Settings at the Personal tab.
Subject - Message subject. You can set default subject it in Supermail Settings at the Message tab.
To - here you can insert email, contact's name or name of contact's group from your Address Book. Check it out- if you've already sent them a supermail, they autofill! Fancy!
Comments - the text of your email.
In the Files tab you can see which files will be sent.
In the Message tab you can set parameters for this email.
You set default message parameters in Supermail Settings at the Message tab.
Metadata tab allow to manage metadata
Once you are done, you can click "Send" or "Save as Draft". If you choose "Later on"- you can find this message in your Supermails in the Unsent Supermails sections.