• Multiple domains using this for email (e.g. @company.com, @othercompany.com, @company-other-spelling.org).
• Webmail on your server (for anyone in the org to access email).
• Aliases / redirects for some email addresses (e.g. so you can redirect “support@” to a particular person).
• DO NOT create “linux users” for every email user – it’s a huge security hole, and a massive pain in the ass for the sysadmin.
• DO NOT do mail-relaying.

• Web server [Nginx]
• Database server (MySQL)
• Email server (MTA) (Exim4)
• IMAP server (Dovecot)
• Webmail server (Roundcube)

The database server will be used to manage ALL logins and usernames/passwords.

You need to install ALL of:

apt-get install apache2-mpm-prefork
(Some of these email servers require PHP; PHP is crappy and requires mpm-prefork (the ‘slow’ version of Apache))
apt-get install mysql-client
(should auto-install something like: mysql-common + mysql-client-5.5)
apt-get install mysql-server
(should auto-install something like: mysql-server-5.5 + mysql-server-core-5.5)
apt-get install exim4
apt-get install exim4-base
apt-get install exim4-config
apt-get install exim4-daemon-heavy
(there’s an “exim4-mysql” that might be sufficient to replace this, but I gave up: there are way too many exim4 packages, and no help for installing the “correct” set, so … just pick this and get the lot!)
apt-get install dovecot-core
apt-get install dovecot-imapd
apt-get install dovecot-mysql
apt-get install roundcube
apt-get install roundcube-core
apt-get install roundcube-mysql

http://t-machine.org/index.php/2014/06/27/webmail-on-your-debian-server-exim4-dovecot-roundcube/

https://weijl.org/virtual-domains-with-exim4-dovecot-dspam-and-mysql/

http://www1.alx.pl/w/linux/exim-sql.conf

http://alex.mamchenkov.net/2010/06/24/exim-dovecot-and-mysql/