While in theory it's true, in practice I've seen multiple systems that due to early implementation bugs they had multiple cases for the same email, which obviou

While in theory it's true, in practice I've seen multiple systems that due to early implementation bugs they had multiple cases for the same email, which obviously were from the same person, think JohnDoe@example.com on one user entry and johndoe@example.com in another user entry. These were also coming from different systems, which made it all even more troublesome. I would argue the risk matrix of having case-insensitive emails looks much better than the risk matrix of having case-sensitive emails (meaning, you should lowercase all the emails and thus The Copenhagen Book is right, once again).