Changing case of email's letters in profile leads to validation error "User with the email already exists."

Take a note that email textbox in registration form is not case-sensitive
Pre-condition: User is registrated
Steps:

1. Go to Profile
2. Change case (for example: "email@yandex.ru" to "Email@yandex.ru") of some letter in current email
3. Click on Save changes button

AR: Can't save profile. Text with validation error "User with the email already exists." appears under email textbox.
ER №1: Email should not be case-sensitive. User can change case of letters in his email and save it.
If user changes case of letters in his email and saves profile - each letter of email in textbox is converted to lowercase after saving of profile.
ER №2: Local-part of email is case-sensitive. We should to add validation note (detect case changing like detect Caps in password textbox) into email textbox in Profile and in registration form. And convert domaine name to lowercase after entering email.
RFC 2821 says:

The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. Mailbox domains are not case sensitive. In particular, for some hosts the user "smith" is different from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged.

For example: The email address ReCipiENt@eXaMPle.cOm is indeed different from recipient@example.com (but it the same as ReCipiENt@example.com).