We have in our shop different wine producers.
When a customer places an order he can choose to buy different wines from different producers.
We need to send an email to each producers who has a products involved in the order.
Example in the basket we have
Wine A -- Producer A
Wine B -- Producer B
Wine C -- Producer A
When the order is placed the system wil send the order detail as defined in the magento rules plus
a mail to producer A with the shipment document for Wine A and Wine C
a mail to producer B with the shipment document for Wine B
The producer mails can be an attribute of the product or an attribute of a category