Like Custom Values, you have against the Customers and Vendors an action called Custom Rules. Custom rules relate solely to the sending of a document type.

In here populate the Document Type Code, if you know the code type it in or use the drop-down arrow to show you the list of document types you have created that you can choose from. The To Address, CC Address and BCC Address

NB: This is optional setup if you only require your document type to be sent to one email address simply setup one email address.

Once completed simply close the page. The setup done above can now also be seen in the custom values page as well as the custom rules page for that Vendor or Customer.