list server returning lots of undeliverables

[JD]

This morning I got about 10 of these:

This is the mail system at host lists.fedoraproject.org.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to<postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                    The mail system

<users at lists.fedoraproject.org>: mail forwarding loop for
     users at lists.fedoraproject.org