[Koha] Where do "bounced" email messages go?

Gentle Reader Following on from my previous email about configuring outbound email in Koha, I have received several helpful replies urging me NOT to "go it alone" and set up Postfix, SPF, DKIM and DMARC but instead to use an SMTP mail relay. (I am considering a UK-based service called Arrowmail.) It looks straightforward to point Koha to a new SMTP server using the staff interface, but I have a dumb question. If the recipient replies to the messages (or the mailer generates an "undeliverable" message) where does the reply go? How do I set the From: header in the message that is generated? Thanks and Best Regards, Chris Brown