Configure postfix to relay all emails through gmail. Tested on Centos 6.4.

gistfile1.txt

#!/bin/bash
#
# Configure postfix to relay all emails through gmail.
#
# Also install mailx to send testmails.
#
# Based on
#   http://mhawthorne.net/posts/postfix-configuring-gmail-as-relay.html
#   http://carlton.oriley.net/blog/?p=31
#   http://opentodo.net/2013/03/postfix-smtp-relay-to-smtp-gmail-com/
#

__author__="daniel.lindh@amivono.com"
__copyright__="Copyright 2013, Amivono AB"


## CONFIG BEGIN

# The gmail account to relay through
GMAIL_USER="noreply@example.com"
GMAIL_PWD="password"

# The email to send test mails to.
TEST_EMAIL="info@example.com"

## CONFIG END


echo "Configure postfix to relay all emails through gmail."


# Requires to be executed by root.
if [ "`whoami`" != "root" ] ;
then
    echo "
ERROR:
You must be root to run this script. You are loged in as \"`whoami`\".
EXIT"
    exit
fi


# Only tested on Centos 6.4, dissalow other dists.
grep "CentOS release 6." /etc/redhat-release > /dev/null || \
(
    echo "
ERROR:
This script is only tested on centos 6.4. It will probably work on other
Centos 6 versions, so that is allowed. But you are running an
\"`cat /etc/redhat-release`\" so you are not allowed to run this script.
EXIT"
exit
)


# Install postifx rpm, usually installed by default.
if ! rpm -q postfix > /dev/null ;
then
    echo "* Install postfix rpm"
    yum install -y postfix
fi


# Make sure postfix has been built with the necessary dependencies.
ldd `which postfix` | grep libsasl > /dev/null || \
(
    echo "* Invalid postfix version" &&
    echo "EXIT"
    exit
)

ldd `which postfix` | grep libssl > /dev/null || \
(
    echo "* Invalid postfix version" &&
    echo "EXIT"
    exit
)


# Install cyrus-sasl-plain to get CAs to accept google certs.
if ! rpm -q cyrus-sasl-plain > /dev/null ;
then
    echo "* Install cyrus-sasl-plain rpm"
    yum install -y cyrus-sasl-plain
fi


# Add postfix config at the bottom of the file /etc/postfix/main.cf.
# The last setting for any option is the one that is saved, so anything
# above this will not be affect these final settings:"
if ! grep "### GMAIL RELAY CONFIG BEGIN" /etc/postfix/main.cf > /dev/null ;
then
    echo "* Configure /etc/postfix/main.cf"

    cat >> /etc/postfix/main.cf << EOF

### GMAIL RELAY CONFIG BEGIN

# Sets gmail as relay
relayhost = [smtp.gmail.com]:587

#
# TLS parameters
#
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy

# Log the hostname of a remote SMTP server that offers STARTTLS,
# when TLS is not already enabled for that server.
smtp_tls_note_starttls_offer = yes

#
# SASL Configuration
#

# Use sasl when authenticating to foreign SMTP servers
smtp_sasl_auth_enable = yes

# Path to password map file
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

# Disallow methods that allow anonymous authentication
smtp_sasl_security_options = noanonymous
smtp_sasl_tls_security_options = noanonymous
smtp_sasl_mechanism_filter = plain

# To accept cert from gmail.
smtpd_tls_CAfile=/etc/pki/tls/certs/ca-bundle.trust.crt
smtp_tls_CAfile=/etc/pki/tls/certs/ca-bundle.trust.crt

# Our box only allows ipv4
inet_protocols = ipv4

### GMAIL RELAY CONFIG END
EOF
fi


# Create /etc/postfix/sasl_passwd file with your GMail login credentials.
if [ ! -f /etc/postfix/sasl_passwd.db ] ;
then
    echo "* Configure /etc/postfix/sasl_passwd"
    cat > /etc/postfix/sasl_passwd << EOF
[smtp.gmail.com]:587  $GMAIL_USER:$GMAIL_PWD
EOF

    sudo postmap /etc/postfix/sasl_passwd

    #
    echo "* Set perimissons on /etc/postfix/sasl_passwd"
    chmod o-r /etc/postfix/sasl_passwd
    chmod o-r /etc/postfix/sasl_passwd.db
    chown root:root /etc/postfix/sasl_passwd
    chown root:root /etc/postfix/sasl_passwd.db
fi


# Create /etc/postfix/tls_policy file with your GMail login credentials.
# Config to force the use of ssl with the gmail smtp server."
if [ ! -f /etc/postfix/tls_policy.db ] ;
then
    echo "* Configure /etc/postfix/tls_policy"
    cat > /etc/postfix/tls_policy << EOF
[smtp.gmail.com]:587 encrypt
EOF

    sudo postmap /etc/postfix/tls_policy

    #
    echo "* Set perimissons on /etc/postfix/tls_policy"
    chmod o-r /etc/postfix/tls_policy
    chmod o-r /etc/postfix/tls_policy.db
    chown root:root /etc/postfix/tls_policy
    chown root:root /etc/postfix/tls_policy.db
fi


#
echo
echo "Restart Postfix and your server are ready to relay through gmail."
service postfix restart


# Setup iptables
echo "* Delete old iptables rules."
iptables -D OUTPUT -p tcp -j postfix_out
iptables -F postfix_out
iptables -X postfix_out

echo "* Add new iptables rules."
iptables -N postfix_out
iptables -I OUTPUT -p tcp -j postfix_out
iptables -A postfix_out -p tcp -m multiport --dports 587,25 -j ACCEPT
iptables -A postfix_out -p udp -m multiport --dports 587,25 -j ACCEPT


# Install mailx rpm to be able to send emails from command line.
if ! rpm -q mailx > /dev/null ;
then
    echo "* Install mailx rpm"
    yum install -y mailx
fi
echo "* Send test mail"
mail -s "Test mail from `hostname`" $TEST_EMAIL << EOF
Just executed install-postfix on `hostname`.
EOF

It works! Thank you!