1.) Edit main.cf

Open /etc/postfix/main.cf and add this lines:

 
 relayhost = smtp.domain.tld
 smtp_sasl_auth_enable = yes
 smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
 smtp_sasl_security_options = noanonymous
 smtp_sasl_mechanism_filter = plain, login

2.) Edit sasl_passwd

Open /etc/postfix/sasl_passwd and add this:

 
 smtp.domain.tld username:password

3.) Run postmap command and restart postfix

Run this line:

 
 # postmap hash:/etc/postfix/sasl_passwd

Now, restart postfix

 
 # /etc/init.d/postfix restart

Important: SMTP server like smtp.googlemail.com don't work!