While installing ZCS you need to disable postfix, since it has own mta running on the server on which you install ZCS. If you forget to stop postfix service you'll get this error while you attempt to send the mail from zimbra mail server.


First test mta service for zimbra

[root@ha1 ~]$  su - zimbra
[zimbra@ha1 ~]$ zmcontrol status
Host ha1.gil.net
        antispam                Running
        antivirus               Running
        ldap                    Running
        logger                  Running
        mailbox                 Running
        mta                     Stopped
                postfix is not running
        snmp                    Running
        spell                   Running
        stats                   Running
        zmconfigd               Running
mta is stopped so, postfix must be running, check it stop.

[root@ha1 ~]# /etc/init.d/postfix status
master (pid  31718) is running...

Now stop it and also from start up.

[root@ha1 ~]# /etc/init.d/postfix stop
[root@ha1 ~]# chkconfig postfix off

[root@ha1 ~]# chkconfig --list  postfix
postfix         0:off   1:off   2:off   3:off   4:off   5:off   6:off