I've edited /opt/iredapd/settings.py according to your guidance, and restarted iredapd.
When testing if this works as intended, I'm getting "Recipient address rejected" on mx.mailserver.com (iRedMail).
/var/log/maillog (iredmail server)
Nov 19 10:18:27 mx postfix/smtpd[1115]: connect from mydomain.com[IP]
Nov 19 10:18:27 mx postfix/smtpd[1115]: NOQUEUE: reject: RCPT from mydomain.com[IP]: 550 5.1.0 <root@mydomain.com>: Sender address rejected: User unknown in virtual mailbox table; from=<root@mydomain.com> to=<webmaster@anotherdomain.com> proto=ESMTP helo=<mydomain.com>
Nov 19 10:18:27 mx postfix/smtpd[1115]: disconnect from mydomain.com[IP]The recipient webmaster@anotherdomain.com is a valid email account/user on mx.mailserver.com (iRedMail).
Thank you.