Supporting POP and IMAP Clients

When you use a front-end server, the names of the servers that host the mailboxes are hidden from the users. Client computers connect to one host name shared by the front-end servers. As a result, moving users between servers is transparent to the users and requires no reconfiguration of client computers.
To log on, a POP or IMAP client sends the front-end server a logon request that contains the name of the mailbox to be accessed. The front-end server authenticates the user and uses Active Directory to determine which back-end server contains the user's mailbox. The front-end server then proxies the logon request to the appropriate back-end server. The back-end server then sends the results of the logon operation back to the front-end server, which returns the results of the operation back to the client. Subsequent POP or IMAP commands are similarly handled.
Note:
SMTP must be available to allow POP and IMAP clients to submit e-mail. You can install SMTP on the front-end server or set up a separate SMTP server. E-mail submission through SMTP on the front-end server works the same as it does on any other server running Exchange. For more information about how to configure SMTP on a front-end server, see Configuring Exchange Front-End Servers.