Using Andreas Mann’s “Schedule Delivery” script (part of his Mail Scripts collection), you can schedule delivery without leaving Mail and without restricting the way you compose your email. You can compose your message as you normally do, without the restrictions of iCal’s Notes field, and add any attachments you want to send.Save it in the Drafts folder and run the “Schedule Delivery” Script (via a hotkey or a quick launcher like Quicksilver). It presents you with a window that allows you to schedule the delivery of all emails in your Drafts folder:

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