I am in need of a solution that will filter smtp email before it gets sent to my outgoing Novell GroupWise server.

Here is what happened last week.

One (or more) of my users had been "socialy engineered" and gave out their email user name and password
Or they were "guessed" because they were not secure.

A spammer authenticated to our email server and sent out spam
Our McaFee email SaaS service caught it and informed me that I need to fix it, or they will stop filtering our email.
The email messages, while logged in as my users, had a "from" in the header as someone completely different.
What I want to do is have something to prevent my users (phone/home computer/outside email client) from sending any email that is "from" any domain other than my own.
Is there a GW setting that can do this? or is there something that I can install in front of the GW system to filter?