Thank you to Linode for these great instructions -

Tested on Ubuntu 14.04LTS

DuoCircle Postfix Configurations

System was configured as "Internet with Smarthost"


[]:25 user:password 

relayhost = []:25

# enable SASL authentication
smtp_sasl_auth_enable = yes
# disallow methods that allow anonymous authentication.
smtp_sasl_security_options = noanonymous
# where to find sasl_passwd
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
# Enable STARTTLS encryption
smtp_use_tls = yes
# where to find CA certificates
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

Side note - some users needed the following configuration options, your mileage may vary depending on the version of Linux you are using. 

smtpd_sasl_authenticated_header = yes
smtp_tls_security_level = encrypt

This was last tested May 12, 2017