Other than using a firewall, is it possible in Domino to configure to block certain attachment file types and by certain users only?
You can block inbound and outbound addresses via the R5 Configuration Document in the Domino Directory. It is under Inbound Controls and Outbound Controls. You cannot block certain files or file types in R5. You would need a 3rd party tool for that. Many virus scanners such as Trend Micro's Scan Mail and Baltimore Technologies MimeSweeper have this capability.