[Mailman-Users] Install question

[Grant Cummings]

I have installed Mailman 2.0.8 via the RPM from Redhat on a server running
Redhat 7.1

The install went fine, web interface works, I can set up new lists, add
users from the website, it mails out a confirmation email, BUT when I go to
send mail to the list, Sendmail bounces back telling me it's not a valid
user.  Sendmail is at version 8.11.6

What have I missed that allows Mailman to create users??

Thanks!
Grant