|In POP3 there is no concept of unread emails. Usually you pull all the messages and delete them, so all new messages are considered unread.|
Some mail servers may change the Status header of each message to mark it as read when the message is fully retrieved. You may try to check that header but it is not guaranteed that your mail server works that way.
If you really need to keep messages in the POP3 server, instead of deleting them, maybe it is better that your application keeps track of message read status in a database table that associates message identifiers with the read status. So all new messages not recorded in that database will be considered unread.