Why can’t I send bounce-backs from a forwarded email address?

In order to send a bounce-back, you must have a registered mailbox that is able to send emails and not just receive them. So therefore, this cannot just be a forwarding address that isn’t able to send emails.

If your address is connected to another Gmail account (like a Gmail alias account), then you must link it as a Block Sender Alias account in order to have the bounce-backs sent. To do this, follow the these instructions.