I am not 100% sure but I may have encountered that email address used for login purposes is case sensitive. Is that correct and if so is that intended?
Live and learn. This article helped me better understand case-sensitivity: https://www.quora.com/Are-email-addresses-.
The reason I brought it up if because I have a user that kept complaining about not being able to login. I assumed it was user error. When I set the user up I configured as firstname.Lastname@domain.com. It was the ‘L’ that created the problem and led me down this path.
As a suggestion you might consider doing what is sounds like google is doing (according to the Quora article) which is ignoring case during login lower(email) but still store as how the user entered the email address. Once you get further outside of technical users, this may save some pain.