Knowing what you are doing
Setting up an email server in Linux is generally quite easy, for those that have the skills, but then we've got to take in consideration that not everyone know how to deal correctly with firewall rules, filtering, certificates, etc...
An easier route would be to use Webmin and the fantastic Authentic theme (https://github.com/qooob/authentic-theme) to reduce the risk of misconfiguring something and open your server to attack.
For those that want an even easier life a product like Collax with 5 users and Zarafa Community could provide a free but business grade all-in-one platform that provides MS Exchange like features ready to be used in about 15 minutes without having to learn a single command (http://www.collax.com/en/products/collax-business-server/overview/).
It's good to "decentralise" the Internet but make sure your servers are configured properly and don't become spam bots or nodes for the next DDoS attack.