1. Setup eZ Publish to use e-mail address as login (probably optional)
2. Register with, e.g., Firstname.Lastname@example.org
Once registered and activated:
3. Login with Firstname.Lastname@example.org and the password => succeed
4. Logout
5. Login with firstname.lastname@example.org and the password => fails

With eZ Publish legacy, the e-mail address has never been case sensitive, except with PostgreSQL / Oracle (EZP-19123).