By convention, email addresses are partially case sensitive. The local part of the email address before the @ symbol is considered case sensitive, according to RFC 2821. Domain names, per RFC 4343, are never case sensitive.
In practice, however, providers typically do not allow two users to register the same account name even if one chooses a different case than the other, even though the RFC allows for this to happen. Providers might also choose to reject incoming email addresses if they are cased incorrectly.
Best practices for email include creating accounts with only lowercase characters and using only lowercase addresses to write correspondence unless specifically instructed otherwise by a recipient.