Courier-IMAP (IMAP protocol)

Courier-imap is an enterprise IMAP server which is fast, scalable and that allows you to use Maildirs.
Courier-imap can easily manage hundreds of thousands email accounts.

With the IMAP protocol (or IMAPS with SSL), you'll be able to sync your email clients with your mail server.
Note that IMAP has the advantage of using a remote folders system. This allows you to synchronize the client with the server.

Unlike POP3 which removes mails from the server when you download them with an email client.
So, IMAP is more interesting than POP3.

Finally, note that there is a package "courier-imap-ssl" which allows you to install the IMAPS protocol (IMAP secured over SSL).