The client had a larger organization with lots of email addresses. They insisted using a self-hosted solution for better privacy and security.
After thorough study we suggested using Zimbra Opensource Edition as the optimal solution, since it is robust and has enterprise grade services and features. And the Opensource Edition is free for non-commercial use.
We chose Ubuntu for the OS as it is on the list of recommended operating systems. Installing Zimbra is an easy and fast process by using its installer script when all services are deployed on the same host.
The configuration is a more tricky process since it has a lot of options. The most important configuration task was to setup backups (using cold backup scripts), SPAM filtering and DOS protection.
The Zimbra installation had minimal downtime and no incidents reported since then. But the free edition has some annoying limitations: no support for warm backups and distributed data store for higher availability.