I want to limit registration based on the domain associated with their email address. I was looking at the
user_register action hook, but it fires after the user is already inserted, which, although it could be hacked into working, is less than ideal. I want to preempt rather than retroactively remove invalid users.
I've looked through the source in
wp-includes/user.php, but nothing in there looks to be helpful. I did notice the
pre_user_email filter, but that doesn't seem to offer any options for doing anything useful since I can't see a way to do anything with that.