Setting up Postfix can be a bit of a jungle of information, I was left scratching my head wondering why Postfix refused to relay an email to an external address even when provided valid user credentials. The issue seemed to be that Postfix SMTPD wasn’t paying any attention to the /etc/postfix/sasl/smtpd.conf file (which contained the auxprop mysql authentication plugin configuration) and hence was falling back to its default PAM based system.

The fix was the most paintfully simple addition to the Postfix main.cf, highlighted bold. This fix seems to only apply to Ubuntu based environments and means that the smtpd daemon will actually pay attention to the contents of /etc/postfix/sasl/smtpd.conf

smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain = $myhostname
smtpd_sasl_application_name = smtpd
broken_sasl_auth_clients = yes


more reading: http://ubuntuforums.org/showthread.php?t=297462