If you want to use Gmail for sending mails from postfix follow this guide.

First, install all necessary packages:

sudo apt-get install postfix mailutils libsasl2-2 ca-certificates libsasl2-modules

If you do not have postfix installed before, postfix configuration wizard will ask you some questions. Just select your server as Internet Site and for FQDN use something you like or if you have a FQDN; use that.

Add the following lines to /etc/postfix/main.cf

relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtp_use_tls = yes

Create a new file /etc/postfix/sasl_passwd to store the Gmail credentials:

[smtp.gmail.com]:587    YOURGMAILUSER@gmail.com:YOURGMAILPASSWORD

Note: If you want to use your Google App’s domain, just replace @gmail.com with your @yourgoogleappsdomain.com

Fix permission and update postfix config to use sasl_passwd file:

sudo chmod 400 /etc/postfix/sasl_passwd
sudo postmap /etc/postfix/sasl_passwd

Next, validate certificates to avoid running into error. Just run following command:

cat /etc/ssl/certs/Thawte_Premium_Server_CA.pem | sudo tee -a /etc/postfix/cacert.pem

Restart your postfix server:

sudo /etc/init.d/postfix restart

Note: you can ignore errors like: warning: /etc/postfix/main.cf, line 43: overriding earlier entry: relayhost=

Test your setup with:

echo "Hello there, this is a test mail from postfix via Gmail." | mail -s "Test" you@example.com

Troubleshooting

If you run into troubles you probably are sending mail from a new IP of from a Gmail address that wasn’t used before.

In maillog you will find something like:
Error: “SASL authentication failed; server smtp.gmail.com”

You need to unlock the captcha by visiting this page https://www.google.com/accounts/DisplayUnlockCaptcha

You can run test again after unlocking captcha.