How to bounce back email to sender

An email client application (such as Mail.app) can't "bounce" anything. Any reply will be just that - a reply, even if it has been doctored up to look like a fake server message.

A real "bounce" occurs when an email server rejects a message, for example if the server does not accept emails from the sender's address, the recipient doesn't exist on the server, or the server is busy. Nothing is going to convince a remote mail server that the mail has "bounced" once it has been received by a client application, since the server already has a record of its acceptance against the mailer ID of the message.

Bouncing an email is folly and Apple removed the feature years ago.

Bouncing an email simply confirms your address and existence.

Moving an email to the trash or making a rule to exclude that sender works silently.