Most Linux distributions that come with Exim, come with sane Exim default configurations. Changing default values and slightly adjusting configuration is relatively easy. On the other hand, building a completely new Exim configuration file from scratch might not.
Exim has by far the most flexible architecture of most commonly used mail transfer agents. As such it is a lot easier to break or do highly complex things.
Due to its modular design, Postfix can be configured to only use parts that are required in each use-case and as such conserves resources and operates fast.
Postfix uses a monolithic, flexible, human-readable configuration file with sane defaults and since it's modular, each module can be used, ignored or switched with another depending on needs.
See discussion at https://groups.google.com/forum/#!topic/mailing.postfix.users/6Kkel3J_nv4 and unofficial repo at https://github.com/vdukhovni/postfix
Postfix is hard to configure: be prepared to spend some time configuring TLS and permissions, through trial and error. Postfix is an awesome MTA but if you are looking for a quick and functional setup, don't use it.
Only MTA out there built on RabbitMQ, a message queeing broker, which means that no messages are stored on the disk, increasing the performance speed of your deliveries as well as reducing unnecessary hardware costs. Setting up in a cluster, both RabbitMQ and MTA will make sure that in case of a server fail, the messages are continued to be delivered and the other cluster picks up the slack.
This product is highly scalable and configurable to your exact requirements. With a friendly Management Console, you can adjust and set all the settings on the go as well as control and oversee your campaigns and clients. The MailerQ keeps building new features which means that customer needs and requirements are taken in consideration to add valuable and good features.