IndiMail is a Scalable, Secure, Reliable, Efficient, and a highly configurable Messaging Platform. Most of the parameters are configurable through set of environment variables or control files. Using a system called 'envrules', these variables are configurable on a per-user basis allowing IndiMail to behave differently for different users (or set of users).

IndiMail has the ability to distribute the users in a domain across multiple hosts. You can have multiple hosts hosting a domain placed anywhere and tied up by IndiMail. This does not force a filesystem architecture like NFS to be used for have large number of users (typically in an ISP/MSP environment)

IndiMail has multiple queues (called a queue collection). The system IDs and configuration files used by IndiMail are all configurable through a set of environment variables, allowing you to customize IndiMail at run time the way you want.

https://github.com/indimail/indimail-virtualdomains