This is an evil error to trace, because it may occur only on deployment, the cause is deep in class, relies on configuration, and is located in utility code. It took me 2 hours.
There is nothing amiss in the mail software or configuration (you may know this already). The issue is in ViewCommon.getForumLink(), as used as a utility by the mail code.
If forum.link property in SystemGlobals is set,
then the property is returned 'null' (not the empty string). This will work for relative addressing, all kinds of JForum activity, but not for email path construction code.
The cleanest solution did not seem to be extra code, nor setting the property to '/'. For example, this property sets the domain name in emails (make it "/" and there will be no domain name). So, even if templating uses relative URLs, even if the action results in different deployment configurations, I'd use the full domain, without a trailing '/',
and email authentication now works.
Joined: Oct 19, 2013
I think saw some earlier posts on problems with email which may have been related to this...