Brian,

Even if the users are using POP3 to access their mailbox they should register an authentication request against a DC (albeit via the exchange server). You should be able to query the LastLogin property of a user account via the WinNT: provider (or the LDAP one if you are lucky enough to have an AD - we are still in the dark ages here). I have some code that will do this if you think it might work for you...

Michael.