Sending mails using postfix to gmail account

I have a problem with using postfix for sending mails to my gmail account. Here is the error that I get in /var/log/mail.log Feb 3 17:15:01 RISH postfix/pickup[14115]: 703D9162871: uid=33 from= Feb 3 17:15:01 RISH postfix/cleanup[14133]: 703D9162871: message-id=20130203114501.703D9162871@noreply.purposenet.com Feb 3 17:15:01 RISH postfix/qmgr[14116]: 703D9162871: from=www-data@purposenet.com, size=3183, nrcpt=1 (queue active) Feb 3 17:15:01 RISH postfix/error[14122]: 703D9162871: to=chandni.247@gmail.com, relay=none, delay=0.14, delays=0.09/0/0/0.05, dsn=4.3.0, status=deferred (unknown mail transport error)

hostname
RISH

hostname -f
RISH

cat /etc/hosts
127.0.0.1    localhost
127.0.1.1    RISH
10.2.8.87 master
10.3.10.154 slave
# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

I need to send a mail from the localhost to my gmail account. Could someone please help me with this..