If you look deeper into this matter we recommand you to take a look into the RFC1460:

ftp://ftp.rfc-editor.org/in-notes/rfc1460.txt

Here ist the listing of the POP commands which you can use debugging purposes for example:

• stat: return basic staticts about the mailbox store
• list: outputs a list of available message in the mailbox store
• retr: returns the message body for the given message id
• dele: delete the given message id
• last: returns the message id of the highest id that has been touched within the session
• noop: no operation
• rset: removes the deletion mark for all emails in the mailbox store
• quit: close connection and after that is done, update the mailbox store