postfix help - Log interpretation

New to mail server. I have been testing postfix and sendmail to see which would suite my needs better. I have postfix up and running. My log files are logging activity which I am concerned about

"Dec 16 08:16:10 postfix/smtpd[15906]: connect from unknown[61.84.118.136]
Dec 16 08:16:11 postfix/smtpd[15906]: NOQUEUE: reject: RCPT from unknown[61.84.118.136]: 450 Client host rejected: cannot find your hostname, [61.84.118.136]; from=<love@yahoo.com.au> to=<shl918@daum.net> proto=SMTP helo=<XX.XXX.XXX>
Dec 16 08:16:13 postfix/smtpd[15906]: NOQUEUE: reject: RCPT from unknown[61.84.118.136]: 450 Client host rejected: cannot find your hostname, [61.84.118.136]; from=<money@yahoo.com.au> to=<skin00@daum.net> proto=SMTP helo=<XX.XXX.XX>
Dec 16 08:16:14 postfix/smtpd[15906]: NOQUEUE: reject: RCPT from unknown[61.84.118.136]: 450 Client host rejected: cannot find your hostname, [61.84.118.136]; from=<tvz@yahoo.com.au> to=<soonang6@daum.net> proto=SMTP helo=<XX.XXX.XXX>
Dec 16 08:16:16 postfix/smtpd[15906]: NOQUEUE: reject: RCPT from unknown[61.84.118.136]: 450 Client host rejected: cannot find your hostname, [61.84.118.136]; from=<tvz@yahoo.com.au> to=<sip2ya-1004@daum.net> proto=SMTP helo=<XX.XXX.XXX>
Dec 16 08:16:18 postfix/smtpd[15906]: disconnect from unknown[61.84.118.136]

Is my server a open relay??
If so how can I stop this spammer?
If the host is rejected, why does he keep trying non stop ??
Can I totally block all access from these ip ranges?, if so how?

Any help would be appriciated, Thanks

This is what I do:
I add them to my /etc/tcp.smtp file. That effectively stops them from connecting to my email server.
The entry syntax is:
xxx.xxx.xxx.:deny ( this in a outright deny, clip IP back per octet as needed, don't forget ending . )
or
xxx.xxx.xxx.:allow,RBLSMTPD="-IP Blocked :: Listed as SPAMMER" ( This will send them a msg that they are a spammer, but I stopped using this when I realized that being nice to spammers left me feeling dirty )
Probably a zombie system is routing to your email server. You may even have a number of them trying you.

I believe you can with IPTables, I'm trying to learn more about them myself, so maybe someone has a good link or tutorial for blocking Spammers with IPTables they could share.

Any help would be appriciated, Thanks

[QUOTE=dba_frog]

I will try these suggestions. Thankyou again, I am most greatful
for the help.