I've tested this with three different email accounts of my own. No email confirmations were received and there is nothing in the spam box.
I've checked to see if my domain is being blacklisted. There is no indication it is.
I've also check the server log for errors. There is nothing.
I'm running an earlier version of XenForo on the same server and have no problems with it, so the issue doesn't seem to be with the host or server.
What could be the problem and how do I fix it?