Hi
I've been having this problem too. (Addresses without a "." in the name are rejected by the list as well.)
The problem occurs when posting user does not have his "name" filled in. So From: "john" john@somewhere.somedomain is ok and just From: john@somewhere.somedomain does not work. No matter if there is "." in the address, opposed to what I said here some time ago. I tried it from different mailers and it behaved exactly like that. I am not sure if <> around the email helps or not, don't remember if I tried it.
As I didn't have time for finding where the problem was I told all people in the list to fill in their names (or at least something) in their mailers/webmails. It solved the problem ;-) It took me quite long though to explain to some people what I wanted them to do :-))
Vladia