Since I am a computer scientist for a living I am somehow pulling a duty by using my own mail system . I have been using iRedMail for a few years now (for personal but sophisticated use). I recently reinstalled iRedMail 0.9.7 in a redundant configuration with very little issues (which could be solved due to the great support here) and overall great success.

Here's what I am using above a standard iRedMail compilation:
- Two vServers virtualized with OpenVZ running CentOS 7 in two different datacenters
- transport encrypted GlusterFS with SSL for Mail Storage (/var/vmail)
- transport encrypted MariaDB Master-Master replication of configuration databases (I am using MariaDB just because I am familiar with it, but one could achieve the same with PostgreSQL and OpenLDAP probably)
- a custom very secure iptables/ip6tables configuration also filtering outgoing connections
- two MX records in DNS (I decided against DNS round-robin and unfortunately with vServers in two different datacenters a failover IP to use e.g. HAProxy and KeepAlived for Load-balancing and Failover was not available and it would have been overkill for personal use anyway and just good for educational purposes only )

For this sophisticated and redundant setup I am paying only 2x7 EUR a month for the virtual servers which is just awesome. Now compare that to Microsoft Exchange