According to RFC 5321, the local-part of email addresses is case-sensitive. However, most providers that I know of (e.g., GMail) are not case-sensitive. (It's actually more complex than that: GMail ignores .s in emails as well.)
Is there a list, or source, of the various rules, including case-sensitivity, for various major email providers? Is there a large-ish email provider than has case-sensitive email addresses?