I’m afraid I’m not quite sure what you’re asking because the registrations aren’t indexed by registrant’s email. Maybe you can explain what you’re looking to do with the attendee/registration records so I can better understand the situation.
We’ll need more details for this, as Josh mentioned, EE doesn’t index by email address and if it’s custom reporting you could be using anything to pull the details in.
If you are pulling in reports using custom code, then it’s your custom code that will need to be changed, but it’s really not clear what it is your trying to do so any answer right now would be a guess.
Can you give an example of where the output is indexed by email?
If you’re using custom code, post that code to a Gist or Pastebin and add the link here so we can see what you are doing and we can go from there.