Let's start with the basics – the POP3 retrieval mechanism is a service that's present on every cas server you rollout – however in order to make things as secure as possible, the service type is set to manual.
Step 1- Set the service to Automatic for every CAS server you with to retrieve pop mail from
Step 2 – Plain text or Encrypted
In The Exchange Management Console, Navigate to Server Configuration, Client Access, choose a CAS server to configure, click on the POP3 and IMAP4 tab, and finally double click on POP3.
Now click on the Binding tab.
The top portion of this dialog represents plain old unencrypted POP3, however, if you've installed a certificate and secured the POP3 service, your port changes from 110 to 005, or anything else that you'd like to specify, should you want to use a non standard port for any reason.
Step 3 – Authentication
The authentication tab presents the user with three choices, which are relatively self explanatory, however I'll each out quickly.
The first option is Plain Text Logon – which is POP3 as you know it, plain text user and password flowing between client and server.
The next option is similar, however requires domain credentials to be passed. Neither one of these options is secure in any way, since credentials are visible as plain text.
The third and default option, it Secure Logon, which forces the client to encrypt the POP3 session before authenticating. This option also requires the name of the certificate used in the TLS transaction to be specified for a successful connection to occur.
Step 5 – Setting limits
The connection Tab allows you to specify limits and fine tune Time-Out Settings, should you suffer from low bandwidth or high latency.
The proxy target port is specified for servers the CAS server may be proxying for, such as exchange 2003 proxy servers.
Step 6 – Determining what comes back
The retrieval settings tab will deliver arguably the most interesting result. The first drop down allows you to determine exactly how a message is retrieved and made visible to the user, the default setting allows Exchange to make the decision for you:
Calendar Retrieval settings default to ICalendar, however alternate calendar URL's may be used, depending if the mail is retrieved inside or outside of your network.
Step 7 – decide who can use it.
POP3 may be enabled or disabled per user. You want to make sure that the users who are meant to use POP can do so, and everyone else should not. In the Exchange Management Console, Double click on the user, click on the Mailbox Features tab and ensure that POP3 is enabled.
However should you wish to change the message format of the retrieved message for a specific user, you may do so at this point by double clicking on the POP3 and un-ticking “Use Protocol Default”