Oddly the error does NOT appear if SMTP Authentication is ON. I had accidentally turned authentication on and could not recreate the problem until I turned it off.

#jdevcon
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The problem with SMTP authentication ON is even worse as there is no indication that there is problem sending the activation link. You get the standard registration msg even though no emails can be sent...

"Message - Your account has been created and an activation link has been sent to the email address you entered. Note that you must activate the account by clicking on the activation link when you get the email before you can login."

In addition, the admin will have no notification that there is a problem as no emails can be sent from the system.

I would make the argument that while technically the end user is registered but not activated it doesn't matter. From their perspective they can't complete registration without an emailed activation link. So, I think the original COM_USERS_REGISTRATION_SAVE_FAILED is appropriate.

genr8r, I hope you saw that there is another PR (4064) that is to be used in conjunction with PR 4065. These two PRs together move the COM_USERS_REGISTRATION_SAVE_FAILED message from one code location to another where it is more appropriate. With the current placement of this message it will be used twice in some error cases related to user registration (and there may be other circumstances where it will be missing altogether).

I happened to find this issue in conjunction with a private extension to the user registration process (some additional information asked during the registration process), and when the checks whether the user made consistent choices failed the COM_USERS_REGISTRATION_SAVE_FAILED message was used twice.

Since I couldn't present this issue together with the private extension I searched for alternative situations where the same thing happens. In doing so I found that with a wrong setting of the mailer used by Joomla! you get the same behaviour.

Regarding the errors when sending mails from Joomla! e.g. in case of registration there are some more cases where the user gets a positive answer in Joomla! while in fact the registration mail has never been sent due to wrong mail setup. Just one example: if for any reason the mailer you configured in Joomla! cannot send mails to other mail domains the mail is nevertheless accepted by the mailer daemon which in turn produces the positive answer in Joomla!. As a last resort the undeliverable mail will be sent to the mail address configured in Joomla! as sender address - but if that mail address cannot be reached too you'll never see that there is a problem sending mails from your Joomla! site.

I don't think you can address this correctly and completely in Joomla!. The only chance is to manually test the whole configuration initially and every time you change it. Maybe there should be a button "send testmail" or the like within the Joomla! server configuration menu for this purpose?