Describe Courier-IMAP Server here.
IMAP and Courier
IMAP servers provide access to existing mail, in existing mailboxes. IMAP servers are not involved in the mail delivery process. You'll still need to have your mail server software receive mail via SMTP and place it in recipients' mailboxes. It does not matter to Courier-IMAP which mail server you use: Sendmail, Postfix, or something else, as long as the mail gets delivered to the correct mailbox. Courier-IMAP just reads mail from existing mail accounts, without needing to know how it gets there. Courier-IMAP uses "maildirs" to store its mail. Please note that this IMAP server does NOT use traditional mailbox files (/var/spool/mail, and derivatives), it was written and optimized for the specific purpose of providing IMAP access to Maildirs. Consult your mail server's documentation for help on how to get it to deliver mail in the "maildir" format. [http://www.courier-mta.org/imap/maildirs.html 
aptitude update aptitude install install courier-imap-ssl
"Maildir" is a directory-based mail storage format originally introduced in the Qmail mail server, and adopted as an alternative mail storage format by both Exim and Postfix. Courier-IMAP is a popular combination with Qmail, Exim, and Postfix servers that are configured to use maildirs. The primary advantage of maildirs is that multiple applications can access the same Maildir simultaneously without requiring any kind of locking whatsoever. Maildir is a faster and more efficient way to store mail. It works particularly well over NFS, which has a long history of locking-related woes.