Is it possible to either clear importance for all mail that goes to a distribution group (so that it's never flagged as "important")? Or to make it so that only some people can flag messages to that distribution group as "High importance"?

Please describe your exchange topology, version

4 mail exchange boxes, 3 OWA boxes, 4 mail store boxes all on Exchange 2003 (6.5 SP 2). Only distribution list members can send/receive to the distribution group, and everyone in the group can mail to it. I hope that answers your question.

I have tested in the lab (Exchange 2003 SP2), theres no feature that can achieve this function in the exchange 2003 You may try to create customized event sink for this task Resources: Exchange SDK documentation How to create VB script or build DLL for event sink How to write event sinks for SMTP and Transport events For developing event sink, Development Forum would be the best place to seek the advice

Thanks for the answer and for the links.