Hi,

Iptables is some of the most misunderstood software around.  It isn't actually a service that can be started and stopped, but rather the user interface for the networking filter in the Linux kernel (net filter).  I am curious as to where this config came from and if that's the running config while iptables is "started".  Starting the iptables "service" on nearly all distros just runs the equivalent  of:

    iptables –F

    iptables –t nat –F

    # Maybe flush some more tables if you have any

    source /path/to/some/saved/rules.sh

The command 'iptables-save' will show you the currently running config to see if it differs from your saved config.  Your rules for the most part look like they should be valid for a mail server.  The only irregularity is allowing tcp port 53.  If your server is using it's own IP as a DNS server (not 127.0.0.1 but like 192.168.0.x or something), DNS is generally UDP traffic so it's possible iptables is killing DNS on the server and mx lookups along with it because you have tcp/53 allowed and udp/53 blocked.