1) Install bsd-mailx
aptitude install bsd-mailx
Select Internet Site in the Postfix Configuration screen.
If you missed it or want do reconfigure do:
dpkg-reconfigure postfix
2) Add mailgun configuration
Edit /etc/postfix/main.cf
relayhost = smtp.mailgun.org smtp_sasl_auth_enable = yes smtp_sasl_password_maps = static:[email protected]:password smtp_sasl_security_options = noanonymous
3) Restart the postifx service
service postfix restart