Postfix-GMAIL.doc

-- No passo 3 altere "you@example.com:password" por seu e-mail e senha separados por ":"; ficando assim: seu_email@gmail.com:senha 
-- In step 3 change "you@example.com:password" to your email and password separated by ":"; looking like this: your_email@gmail.com:password

-- No passo 9 Substitua you@example.com pelo seu e-mail no seguinte código e teste o envio de e-mail.
-- In step  9 Replace you@example.com with your email in the following code and test sending mail


1 -  install postfix and mailutils:
apt-get install postfix mailutils libsasl2-2 ca-certificates libsasl2-modules -y

2 - Create your password file with: 
nano /etc/postfix/sasl_passwd

3 - Populate the password file. Example: 
[smtp.gmail.com]:587 you@example.com:password

4 - Secure the file by running:
chmod 600 /etc/postfix/sasl_passwd

5 - Replace the contents of the config file by running:
nano /etc/postfix/main.cf

6 - Place the following in the file:

relayhost = [smtp.gmail.com]:587 
smtp_use_tls = yes 
smtp_sasl_auth_enable = yes 
smtp_sasl_security_options = 
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd 
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt


7 - Encode password file by running:
postmap /etc/postfix/sasl_passwd

8 - Restart postfix:
systemctl restart postfix.service

9 - Replace you@example.com with your email in the following code and test sending mail: 
echo "Test mail from postfix" | mail -s "Test Postfix" you@example.com
 
 Fonte: https://serverfault.com/questions/510251/postfix-gmail-authentication-required-error