"Relaying not allowed" that's an important thing you said there ;-)
in your main.cf you define the "mydestination", the value of that setting will tell postfix which domains have theire email hosted on the server. For example:
mydestination = localhost, foo.tld, bar.tld
That means, the server will accept email for localhost, foo.tld and bar.tld
now if you want to send an email to "LinuxGuyMT@foo.tld" the message will be delivered on the server.
but if you'd send an email to MarkNL@other.tld
.. the server checks mydestination, and notices "other.tld" is not in that list, so it will try to find the server that hosts "other.tld" and will send it to the correct server.
That is called "relaying". Relaying is only allowed if the person sending that email authenticates himself with the server. (unautorized relaying is disabled by default to prevent spammer from abusing your mailserver to send spam)
So in short: When you want to send mail via your mailserver, enable SMTP authentication and use your user/pass, problem solved
p.s. if you want to get rid of that "hacker", block him in iptables (firewall):
Block: iptables -A INPUT -s 126.96.36.199 -j DROP
Remove Block: iptables -D INPUT -s 188.8.131.52 -j DROP