When you hit the Supermail button you'll see:

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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.

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In the Message tab you can set parameters for this email.

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You set default message parameters in Supermail Settings at the Message tab.

Metadata tab allow to manage metadata

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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.