ok, i admit the rfc may prescribes something like .+(@.+)? i've just drawn the conclusion from that the local part is interpredet solely by the receiving party

ok, i admit the rfc may prescribes something like .+(@.+)? i've just drawn the conclusion from that the local part is interpredet solely by the receiving party. that's why case-conversion should be avoided, since the receiving party may be case sensitive. so further thinking, local part can be empty perhaps, depending on the implementation.