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Supporting POP and IMAP Clients

 

Topic Last Modified: 2005-05-24

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.

noteNote:
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.

POP and IMAP e-mail clients send user and password information in clear text. If the front-end server is accessible from the Internet, you should configure SSL so that user authentication information and data is not passed over the Internet in clear text.

When a non referral-enabled IMAP client connects to a back-end server, it can access only public folders that have a replica on the user's home server. To access public folders that have replicas on other servers, an IMAP client must be referral-enabled. A referral-enabled client issues special commands to an IMAP server to create a list of the public folders available to the client. When the client computer requests a public folder that does not have a local replica, the server responds to the client request with a referral URL that contains the name of the server that has the public folder. The referral-enabled IMAP client computer then creates a new connection to that server to retrieve the appropriate information.

In a front-end and back-end topology, however, the front-end server acts as a referral-enabled client, so IMAP clients connecting to the front-end server do not need to support referrals; the front-end server handles referrals for them. It transparently maps non referral-enabled client requests to their referral counterparts, making the entire list of public folders available to a non referral-enabled client. When the front-end server receives a referral response from the back-end server, it does not pass this response back to the client. Instead it follows the referral for the client and makes a connection to the appropriate back-end server that has the data. The back-end server then responds with the requested item, which the front-end server relays back to the client.

 
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