Distribution List: Doesn’t Work

Inherited this Exchange 5.5 installation. Most of the time it works o.k., however there’s something wrong with Distribution Lists.

Create list in ExchAdmin. Add members. It shows up in GAL, and in company Outlook address books. You can choose the group as a recipient from all Outlook 2000 clients – but when you attempt to send a message, it just (1) stays stuck in the outbox (some clients), or (2) message appears to send, dissapears completely (other clients).

Any ideas? Clues to troubleshoot?