For an SMTP server which does not use authetication, the correct config.inc.php setting appears to be to comment out smtp_auth. However, this is not obvious as the right thing to do.
My suggestion is that there should be an explicit option, like smtp_auth = NONE. If this is described in the comment above smpt_auth in config.inc.php, it should make it clear enough how to handle this SMTP server configuration. This should be suitable for people like me who try to get things to work by trying all settings.
This suggestion is a result of me taking some time to do something simple. See viewtopic.php?f=3&t=3138
Thank you for this nice software.