Make sure you check Enable Templated Email in the product's settings, if you want the Email Message you type there to replace our default message and allow the use of template variables like [%first_name%] in your message. Make sure your Email Message template includes either [%thankyou_link%] (inserts a link to the thank-you/download page for all items in the buyer's order) or [%download_link%] (inserts a link to a thank-you/download page for just that product).
If Enable Templated Email is not checked, the Email Message you type for the product cannot use template variables and will simply be added to the default message we normally send for digital products ("Dear Firstname" followed by brief instructions and a link to reach the thank-you/download page for that product), rather than completely replacing that default message, which sounds like what you described is happening.
