MSExchangeTeam.com has published a useful post explaining how to set up POP and IMAP in the Exchange Server 2007 system.
Most people these days are switching to Outlook Anywhere (RPC over HTTP), Outlook Web Access, or Exchange ActiveSync because they provide a richer client experience than POP or IMAP. Certainly, I use these methods when I am not at my primary corporate desktop machine. However, there are still those who are in the world of *nix or who have a client or phone that only supports POP or IMAP protocols.
Configuring Exchange 2007 for POP3 and IMAP4 clients is a little more complicated than in previous releases. The two major reasons are the introduction of server roles and the current lack of GUI for POP3 & IMAP4. The complications imposed by lack of a GUI should be solved in Service Pack 1. In this post, I will help you get up and running with POP3 & IMAP4 by helping you to configure the Exchange 2007 server and a sample client.
POP3 and IMAP4 protocols are part of the Client Access Server role. However, SMTP is also required for these clients to be able to send mail. SMTP is part of both the Edge Transport and Hub Transport server roles. Although Edge Transport server role is recommended for the Internet-facing servers in your organization, Hub Transport is more suited for authenticating and providing SMTP relay services for POP3 & IMAP4 clients, as Edge servers are generally not connected to the domain. If the Edge server is not part of the domain, it cannot provide user-based authentication as easily.
Read more at source: http://msexchangeteam.com/archive/2007/05/16/439093.aspx