Email addresses are terribly confusing if you've ever read the RFC:
http://haacked.com/archive/2007/08/21/i-knew-how-to-validate-an-email-address-until-i.aspx

Technically speaking if I read everything right the first part of the mail address (the smith in smith@aol.com) IS case sensitive.

However, even in the RFC (http://tools.ietf.org/html/rfc2821) states that following the exact case is discouraged:
"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."


References:
http://email.about.com/od/emailbehindthescenes/f/email_case_sens.htm
http://email.about.com/cs/standards/a/email_addresses.htm