Whenever someone registers, this message shows up:

You are now registered. However, we were unable to send the activation mail to your email account due to an internal error that had occurred on our server. We are sorry for the inconvenience, please send the webmaster an email notifying him/her of the situation.


When they log in, this shows up:

The selected user has been deactivated or has not been activated yet.
Please contact the administrator for details.
If the page does not automatically reload, please click here

I can still activate anyone manually by trying to edit a user and activating it from there.
Help?

Check your mail settings in Admin/Preferences. See what mail delivery method you are using. You might try the other options. Search the FAQ's also, I'm sure it's been asked before. Hope that helps some.

Also check this thread:

https://xoops.org/modules/newbb/viewtopic.php?topic_id=53370&viewmode=flat&order=ASC&start=0