POP3 is a good Mail Delivery Agent for most situations. However, if you like your mail controlled from a central server, with access to all your mail and folders from a remote site regardless of whether your mail is stored locally, then IMAP is the answer.
IMAP is a connection-based protocol, which means that you need a connection to the server to read your mail. Some MUAs enable you to read your mail locally and offline while not connected, but the server holds the up-to-date representation of your mail.
There are two main contenders in the Unix IMAP server space, University of Washington and Cyrus IMAPD. Both IMAP servers are extremely good at what they do, and we have chosen Cyrus as our focus because it is the IMAP server of choice by SUSE (it is the IMAP server used in OpenExchange).
The Cyrus IMAP server is extremely scalable and is capable of handling thousands of mail accounts. If more scalability is needed, you can use a Murder of Cyrus (a cluster of Cyrus servers) to provide a distributed IMAP server farm.
To use Cyrus, you need to add a system user (in our case, we will use justin) to the Cyrus database. All authentication is still handled through the user's system password, but the Cyrus database needs to provide a namespace for the user, including folders and access control lists.
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