Quote:
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Firewall Configuration should be set to No firewall, and the Security Enhanced Linux (SELinux) should be disabled.
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taken from
Zimbra™ Collaboration Suite Single Server Installation - Open Source
I would check your DNS setup for the other issue
Code:
dig _domainname_ mx
dig _domainname_ any
dig _fqdn_hostname
cat /etc/resolv.conf
cat /etc/hosts