In the 
Invoice Settings of the Invoicing module, you can define default email addresses to which you want to send a copy of all invoices.
Adding a BCC address when sending invoices can be useful for sending a copy of all your invoices to the accounting system.To set this up, go to the advanced settings of invoices and then to "Default Email".
Here you then have the option to enter one or more email addresses to which you want to send a copy of invoices, if they are sent by email.
If you want to enter multiple email addresses, they must be split by a ; as also indicated when you hover over the question mark. You shouldn't add extra spac