Is there a User exit / BADI which could be used to send e-mails to multiple addresses...
There will be no User-Exit or a BADI with a predefined functionality. You can include a FM that sends mail in any of the BADI's or User exits.
Basically BADIs or User exits are places where you can write code customizations as the Process requires. You need to include your own code there. In there you can include a FM to send mail.