to be fair, the email RFC is a silly document that does silly things. The user part of an email address "may" be case-sensitive.
But in practice, no one does that. Because imagine grandma on the phone with her bank not remembering if her email address was Mary123@gmail.com, MARY123@gmail.com, or mary123@gmail.com. The horror.
Some systems (used to) map the local part to local unix usernames, which are case-sensitive. Hence the sender would get a 550 No such user (or similar) if he got the case of the local part wrong.
But in practice, no one does that. Because imagine grandma on the phone with her bank not remembering if her email address was Mary123@gmail.com, MARY123@gmail.com, or mary123@gmail.com. The horror.