You can use the following 2 options to identify the POP3 users in an Exchange environment.
1. Enable diagnostic logging for the POP3svc service for authentications and connections. You will see the connection details in the event log.
2. Another option is to view pop connections under your pop SMTP virtual server which is located in Protocols -> POP3 -> Default POP3 Virtual Server -> Current Sessions.
Also, a log file will be generated in the Exchange logging folder under POP3.