outbound mail failure from <>

As administrator I am receiving 30 or 40 outbound mail failures each day that are obviously spam from user <>, and less frequently from spoofed user names. Specific message is this: Notification: Outbound Mail Failure – Message timed out.

Does anyone know how I can prevent this? I fear I am the victem of a spammer. We are running MS Exchange 5.5 sp 2 on NT4.0 sp 6a.