If we want to check when a user last logged on to their mailbox in Exchange 2007 and 2010 we have to use the Exchange Management Shell [EMS] and the following cmdlet:
Get-MailboxStatistics "User" | Select LastLoggedOnUserAccount, LastLogonTime
Note that the LastLoggedOnUserAccount property indicates the account last used to log on to the mailbox. This could be a user with FullAccess permissions to the mailbox, a delegate or even someone simply checking the user’s Calendar!
With Exchange 2013 part of this information is now available through the Exchange Administration Centre as well. If you:
1. Navigate to Recipients and then Mailboxes;
2. Double-click on the user you want to check this information for;
3. Select the Mailbox Usage tab and in there you will see the Last Logon date and time.