Well that’s wrong. Hostnames are case-insensitive, but usernames in email should be case-sensitive.
An address normally consists of user and domain specifications. The
standard mailbox naming convention is defined to be
“local-part@domain”; contemporary usage permits a much broader set of
applications than simple “user names”. Consequently, and due to a
long history of problems when intermediate hosts have attempted to
optimize transport by modifying them, the local-part MUST be
interpreted and assigned semantics only by the host specified in the
domain part of the address.
Key point: the local-part MUST be interpreted and assigned semantics only by the host specified, which means you are not allowed to make any assumptions (like case sensitivity) about it.