I’m using CentOS 6.
Here are the two that I narrowed down to, other suggestions are welcome.
https://digitalocean.com/community/articles/how-to-install-iredmail-on-centos-6-3-x64
OR
https://digitalocean.com/community/articles/how-to-set-up-a-postfix-e-mail-server-with-dovecot
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The default MTA on a CentOS 6 is Postfix. That would be an easy setup than iRedMail, but besides resources you need to consider your tech skills and needs when it comes to manage your email server.
Compare the RAM requirements of the two. That’s should help you come to a better decision.