Followed the mailgun documentation to get SASL relay working but was getting

SASL authentication failure: No worthy mechs found

Also found that the default configuration wasn't finding the ca-bundle.crt

# /etc/postfix/main.cf
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = static:[email protected]:yourmailgunpassword
smtp_sasl_security_options = noanonymous
# add this line to point to ca-bundle
smtp_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt
smtp_tls_note_starttls_offer = yes
smtp_tls_security_level = may
smtpd_tls_CApath = /etc/pki/tls/certs
smtpd_tls_cert_file = /etc/pki/tls/certs/smtpd.pem
smtpd_tls_key_file = $smtpd_tls_cert_file
smtpd_tls_security_level = may

Also discovered that there was a missing module

yum install cyrus-sasl-plain

http://priority-zero.blogspot.com.au/2013/03/postfix-with-gmail.html

Creating a self signed cert

cd /etc/pki/certs
make smtpd.pem