Courier-pop is a POP3 server created by Double Precision that will allow your users to access their emails by using an email client.
Important : by default, email clients erase emails from the server when the user logs in to his e-mail account using the POP3 protocol. To avoid this problem, change the client configuration or install the IMAP protocol on your server to prevent your users experience this problem.
Note : Courier also provides a "courier-imap" package that will allow you to install the IMAP protocol of Courier.
Finally, note that there is a "courier-pop-ssl" package which allows you to install the POP3S protocol (POP3 secured over SSL).