[Mailman-Users] problem with cron

[nikos]

Hello list.
I've just install mailman 2.1.9 in a fedora 6 server.
Everything works fine except cron.

I'm recieving errors like:

You are not allowed to post to this mailing list, and your message has
been automatically rejected.  If you think that your messages are
being rejected in error, contact the mailing list owner at
mailman-owner at myHost.

and subjects:


Cron <mailman at mail> mailman /usr/lib/mailman/cron/nightly_gzip
Cron <mailman at mail> mailman /usr/lib/mailman/cron/checkdbs

What did I do wrong?
I've just load contab.in in crontab -umailman
What will happents If I stop cron for mailman?

Thanx
Nikos