It’s that capitalization is language dependent, which email addresses shouldn’t be as I hope the rules for France shouldn’t be different than for Dutch. For instance é in Dutch is capitalized as E, but in French it is É. The eszett didn’t even have an official capital before 2017
In most programming languages, case-insensitive string compare without specifying the culture became deprecated. It should imo only be used for fuzzy searching doubles, which you probably will do with ToUpper for performance reasons, or maybe some UI validation.