I said that I would try to install over 2 million users onto another email system and the one that I used is iRedmail. It is a system based on Postfix, Dovecot, Roundcube and Amavisd that is installed with a simple script. You will not find a load of un-needed features on it, because it has been designed to just get the job done, rather than to be bloated out with features, so that some salesmen can try to extract even more money from some over-rated enterprise companies, who were helped by slavery. The system had no problems with the over 2 million users, but I did have to add a Codeigniter Datatables interface on top of the database, because the iRedmail interface does still lack a searchable user list.

The SQL files that I used to load up the users into iRedmail can be downloaded from the below link:

https://drive.google.com/open?id=0B5OhdvjzzVvea1VUSEg0SFQyUzg