My site (currently http://www.goodmangenealogy.org) is hosted on GoDadddy, and I knew from past experiences that they require smtp to submit all outgoing emails from WP websites hosted on their servers. So I configured cforms accordingly, using the same smtp settings and logins that I had successfully used elsewhere, including my Outlook email client. It didn't work.

For cforms settings, I checked, confirmed and tried absolutely everything posted in the FAQs and in this thread, all to no avail. Using Firefox with Firebug and its console debugging option, I saw that no matter what I did, I got "authentication failed" response.

I deactivated this plugin and tried another one, with similar issues. However, that plugin author provided a specific FAQ for Godaddy hosted sites, which mine is, and hat finally solved the problem for both plugins.

See: http://www.fastsecurecontactform.com/godaddy-email

First, I installed and configured the WP Mail SMTP plugin they recommended, configured it as specified in their FAQ, and tested it. It Worked!

Then I then deactivated the WP Mail SMTP plugin, reactivated cforms, and applied the same exact smtp and email settings to my cforms smtp setup, but it still did not work.

Then I reactivated the WP Mail SMTP plugin, changed the cforms global email settings to use the standard WP php mail function instead of smtpout, with no other changes to anything in cforms, submitted a cforms test contact form, and voila, that worked!

So, problem solved for me, as long as I use the WP Mail SMTP plugin. But there is apparently still some kind of smtp bug in cforms when attempting to authenticate with the Godaddy smtp server. With this solution, and perhaps a review of the working smtp authentication code in the WP Mail SMTP plugin, maybe it can be fixed. Or, maybe cforms should simply specify the use of the same the WP Mail SMTP plugin, strip out its own internal smtp code, and let someone else (another plugin author) be responsible for all smtp server compatibility issues.