When installing and configuring Zimbra for the first time on RHEL/CentOS 6, it return that LDAP failed to initialize and then the config script exit.
The log file probably indicate the following error :
*** Running as zimbra user: /opt/zimbra/libexec/zmldapinitsudo:
sorry, you must have a tty to run sudo
IO::Socket::INET: connect: Connection refused at /opt/zimbra/libexec/zmldapinit line 137.
This is because the sudo config file had an option enabled by default to deny any sudo command for user not logged in a real TTY. So you may need to temporarily disable this feature by editing /etc/sudoers and comment the following line :
# Defaults requiretty