You can look at the GWIA logs to see whom is being sent and receiving internet (external) mail if you look at the Agent Accounting Data File. I have set up so that it e-mails the administrators automatically you can extract the file into excel or whatever you want to make it more readable. Or you could sit at your server and watch the GWIA!

Another way is to reset everyones password and see who moans! (Not a very good idea) And would not work if people were using proxy to see a second account.

:0)

The main problem with watching the account traffic is that people no longer there usually neglect to sign off of any Listservs when they leave. And there is always Spam going to the addresses too. I regularly find mail boxes full of junk long after people depart. You may need to work with HR and compare their Employee list to the GW Address book then check to see the Last Login in NDS.
I have Departments that just set the old Account to Auto Forward messages to the Manager or a new Employee and forget about it. Eventually the Sent Items fills up with a lot of junk.

For what it's worth,
Ken