I am having a problem as well, but I don't know if it is the same as yours. I was just setting up my first discussion list yesterday. When a new user replied to the list-request email, that reply would bounce with the following error:
----- The following addresses had permanent fatal errors -----
(reason: 550 email@example.com: Recipient address rejected: User unknown in virtual alias table)
(your "list" and "example.com" will likely be different)
The same thing happens when I try to send an email to the distribution list. A quick web search suggests it is a postfix problem (see http://www.freebsddiary.org/mailman-virtual-domains.php) but I don't think we have access to the postfix configuration files. Even if we do, I am not educated enough to figure out what I should be doing.