This will explain how-to setup, Winbind to authenticate against Active directory to use Virtual Users, Postfix as the MTA, Spamassassin & Procmail as the spam agent, SquirrelMail as the MUA and the autosuscribe plugin to hold all the junk mail & Dovecot as the imap server.

This is something I have just had to setup, it's mainly parts from other how-to's all put into one.

Name: Dovecot
HomePage: http://dovecot.org/
Function: Dovecot is an open source IMAP and POP3 server for Linux/UNIX-like systems, written with security primarily in mind.

Name: Postfix
HomePage: http://www.postfix.org/
Function:  Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users

Name: Squirrel Mail
Homepage: http://www.squirrelmail.org/
Function: Webmail client, used to check e-mails from anywhere in world, via the internet, like gmail or hotmail

Name: Spamassassin
HomePage: http://spamassassin.apache.org/
Function: SpamAssassin is a mature, widely-deployed open source project that serves as a mail filter to identify Spam

Name: Procmail
HomePage: http://www.procmail.org/
Function: procmail is a set of rules for each mail that comes into the system.

OpenSSL will be used to make the ssl certificates used by the vsftpd server. The net2ftp will be used to connect to the vsftpd server to transfer files across the internet. The programs used in this guide will be:

Name: vsftpd (Very Secure File Transfer Protocol Daemon)
HomePage: http://vsftpd.beasts.org/
Function: the simplest and most secure way to exchange files over the Internet

Name: OpenSSL
HomePage: http://www.openssl.org/
Function: developed for transmitting private documents via the Internet using cryptographic a system

Name: Apache
HomePage: http://www.apache.org/
Function: Webserver

Name: Net2FTP
HomePage: http://www.net2ftp.com/
Function: a secure FTP client