> I also installed a few months back an application > that restricts, which domain name a user can use
> to email out of.
That will most likely be the problem, one of the mailrules variants, perhaps
e-smith-domain_mail_user-0.4-1.noarch.rpm
or
dungog-mailblocking-1.0-3.noarch.rpm
Try
rpm -q e-smith-domain_mail_user
rpm -q dungog-mailblocking
The workaround is to put the mail list name in the "allowed from" list.
Those rpms are blocking mail for invalid email addresses, and they do not seem to be aware of the newly added mail list address.