Thank you to Linode for these great instructions - https://www.linode.com/docs/email/postfix/postfix-smtp-debian7
Tested on Ubuntu 14.04LTS
DuoCircle Postfix Configurations
System was configured as "Internet with Smarthost"
relayhost = [outbound.mailhop.org]: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