Preserve old UserEmail

[jace]

When a user removes an email address (which was verified), preserve it in a new UserPastEmail (or similar) model, with no unique constraint on the email address.

Past emails can be a useful indicator of past associations.

[miteshashar]

If there would be an email address existent in UserPastEmail, would new accounts with that email address be allowed?

[jace]

Yes. This is simply a log.