• 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.

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/