I believe you must be referring to the Notebook app made by Appigo. That app syncs with the Toodledo Notebook which you can access by clicking on "Notebook" in the left page frame. You cannot add directly to the notebook via email. Only tasks can be added that way. Here is a link to Appigo's site for more information: http://www.appigo.com/
If you want to be able to add tasks by email, and get them on the iPhone, you will need a task manager that syncs with the site like the official Toodledo app, or Todo which is also made by Appigo and work with their Notebook app.
To get something to show up in the iPhone Notebook app, you can either add it directly from the website, or even cut and paste the notes from the tasks you added.
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