[Mailman-Users] not allowing user not in system to send

[Kevin Carlyle]

Jul  2 20:36:32 374liberty postfix/smtpd[9839]: reject: RCPT from
localhost[127.0.0.1]: 554 <username at kc.rr.com>: Recipient address
rejected: Relay access denied; from=<fireball-bounces at 374liberty.org>
to=<username at kc.rr.com>

I'm getting that and with some testing discovered that it's because the
system does not have a user by the name of fireball-bounces, just
fireball.

how do I setup the email system to allow emails with that address without
adding an account in the system?

Regular email (short of aol, I'm working on that) works fine.

I'm using postfix.