Event ID 1016 Event Source MSExchangeIS Mailbox Store Event Type Success Audit Event Category Logons Description Windows 2000 User <domain\user> logged on to <mailbox address> mailbox, and is not the primary Windows 2000 account on this mailbox.
Note You can run the set-eventloglevel command in Exchange Management Shell to enable logging against user logons. This command resembles the following:
set-eventloglevel -identity "MSExchange IS\9000 Private\Logons" -level low/medium/high/expert