Note that the local mailbox portion MUST BE treated as case sensitive in the context of the SMTP protocol and such, but it is up to the mail host to actually de

Note that the local mailbox portion MUST BE treated as case sensitive in the context of the SMTP protocol and such, but it is up to the mail host to actually define the semantics of the local part. Non-case sensitive mailboxes is not prohibited anywhere in the spec, what you are referring to is there to prevent the local portion of the address from being changed when passed through a relay. "the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address."