Are e-mail addresses case-sensitive?
Yes, according to RFC 2821, the local mailbox (the portion before @) is considered case-sensitive. However, e-mail addresses are not usually case-sensitive because of the difficulties and confusion it would cause between users, the server, and the administrator. Therefore, when sending an e-mail, it's safe to assume an e-mail address like "[email protected]" is the same as "[email protected]."
Realize that the username associated with your e-mail account is very likely case-sensitive. For example, if your username was "support," you would not be able to log into your account using "SUPPORT" even though e-mail can come through as "[email protected]."
As a general rule, when creating an account username we always suggest using a lowercase username.