That's a problem with your mail server; you'll most likely need to contact your host about it.
Search is your friend “I was having a mildly paranoid day, mostly due to the fact that the mad priest lady from over the river had taken to nailing weasels to my front door again.”
The product is not sending emails back to registrants and I am not sure why, but it appears we will have to scrap and go to another product if it can not email.