Open source alternative for microsoft exchange
 

There are so many different open source Microsoft exchange/365 alternatives out there (Zimbra, Open X Change, Kolab, Citadel). I want to know, out of all of the open source Microsoft exchange alternatives that are out there, which is the most secure (suffers lesa from vulnerabilities) but also provides with the most features that microsoft exchange has.

Right now I have Postfix/Dovecot with roundcube as my mail server (running centos 6.5) and it does the job (except has no calendar support).

security is a lose word to use. how well do you understand firewalls, how well do you understand IPTables and SELinux, what other services will you be running etc?

If you can migrate to CentOS 7 when it releases, probably sometime later this year (id guess late 3rd to mid 4th quarter) that will help greatly too. loads of improvements in RHEL 7 over 6.

Sadly the only one you have listed I have any experience with and it was only end user is Zimbra. It was handled semi well. The issue was the county (Seminole County in Central Florida) was running it on old hardware running MS Windows server, not RHEL as they should have been. Thanks to the money donated to the county by Microsoft to prevent FOSS from running in their network.

I know what firewalls do and what iptables and selinux are. By security, I meant which one has the least amount of vulnerabilities or potential vulnerabilities. I guess I can give Zimbabwe a try. Thank you.