Do you want your servers to send mails with a dedicated mailing service in order to gain more control and logs about the mail delivery?
Powerful APIs that enable you to send, receive and track email effortlessly. 10,000 emails free every month. mailgun.com
If you already have postfix properly set up and running, edit
/etc/postfix/main.cf and append this:
smtp_sasl_auth_enable = yes relayhost = smtp.mailgun.org smtp_sasl_security_options = noanonymous smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
Then add your smtp credential of mailgun to postfix:
echo 'smtp.mailgun.org firstname.lastname@example.org:password' > /etc/postfix/sasl_passwd
Protect the file and populate it:
chmod 600 /etc/postfix/sasl_passwd postmap /etc/postfix/sasl_passwd
And finally restart postfix:
service postfix restart
Now your server should send its mails via mailgun. Just give it a test shot!