We do have a report that will show what users access there account via IMAP and then you can look at the IMAP log to find out the IP address they are coming from.  The report is under Reports | System Summary Reports | Traffic Reports | IMAP Usage.  There it will break it down by domain and if you click on the domain it will then show by users.  Once you have the users go to Manage | View logs and change your type to IMAP (need to be set to detail) and you can then search for the email address and find what IP address they are connecting from.