New York Times says:
The domain-name portion of the e-mail address - the part after the @ sign - is not case -sensitive, but the first part of the address may need to be typed in exactly as given. This rule is explained in RFC 2821, the document laying out Internet e-mail standards, found at faqs.org/rfcs/rfc2821.html.This certainly is news to me. I never had thought that my username was case sensitive. Apparently, in most cases they aren't. The RFC2821 states:
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".But then, I am not the only one who remains in this blissful state of ignorance. Hundreds of thousands of management aspirants who filled out the CAT application forms to India's premiere B-schools just last month would have noted that the email address field had only uppercase options!