Yes we have tried this authentication method. We first got the following
Apr 14 11:43:29 xxxxxxx postfix/smtpd[17754]: lost connection after STARTTLS from discourse.xxxx[XX.XXX.XX.XX]
Apr 14 11:43:29 xxxxxxx postfix/smtpd[17754]: disconnect from discourse.xxxx[XX.XXX.XX.XX]
Apr 14 11:43:29 xxxxxxx postfix/smtpd[17756]: lost connection after STARTTLS from discourse.xxxx[XX.XXX.XX.XX]
So I have changed the value of DISCOURSE_SMTP_ENABLE_START_TLS to false in the discourse app.yml file and I rebuiled it. Then, we got the following in our postfix server logs:
Apr 14 12:02:30 xxxxxxx postfix/smtpd[17754]: connect from discourse.xxxx[XX.XXX.XX.XX]
Apr 14 12:02:30 xxxxxxx postfix/smtpd[17754]: warning: discourse.xxxx[XX.XXX.XX.XX]: SASL LOGIN authentication failed: Invalid authentication mechanism
Apr 14 12:02:30 xxxxxxx postfix/smtpd[17754]: lost connection after AUTH from discourse.xxxx[XX.XXX.XX.XX]
Apr 14 12:02:30 xxxxxxx postfix/smtpd[17754]: disconnect from discourse.xxxx[XX.XXX.XX.XX]