Does this ADSI stuff work if you don't have the LDAP setup on your network?

I have an interesting problem:

Some users NEVER log on to our network but get email via pop3, so it is very difficult to figure out what accounts are active or not. (I don't make the policy!!!) This is compounded by the fact that McAfee GroupShield runs on our exchange server and logs in every couple of weeks when we run an "on-demand" scan of all our mailboxes, so it shows the last logon as being the McAfee and not the user... so.. does anyone have a solution for finding out which users have been active within the last 60 days, even though they do not always log in through our domain controllers?

Brian