I did look for information and saw this thread at: https://www.sitepoint.com/community/t/check-if-already-in-database/23935 but I would like the opposite - check if email address is NOT in the system or on WP users list.
I've tried finding information but couldn't find it so far. I'm building a membership site along with registration and login system and members only event registration in Wordpress. It has email address checker to check if the user is in the system and it works fine but it's not what I'm looking for. I'd like to know if there's something that checks any incoming email address and check if it's in the system. If it's not, then prevent the registration getting through by leaving error message that the person needs to join as a member first in order to register for event. Does anyone know what's the php code that simply checks email address NOT in the system? Thanks.