Works like a charm thank you. What would be the best way to only send these e-mails if invoices are created by the subscription module? Because now I would receive an e-mail when I create an invoice myself too, right? Also, I just received an e-mail for all existing invoices :)

Yes, the answer just shows the simplest way and if it just like this, it'll not work properly... because it will be trigger for creation and modification. so for complex situation you need to write a python code to the action ... even so, you can make a simple workaround by add a key word like "recurring invoice" to the source document of the subscription which it'll be the invoice in the "Additional Information" field ,then in your automated action make a new "before update filter" and choose the model to be "Invoice" and make a domain like [('comment','=','recurring invoice')]. Thanks....

It works like a charm to me, but i need other feature: how can i send this mail to a group?