The response code. Must be one of MilterConstants#SMFIR_CONTINUE, MilterConstants#SMFIR_REJECT,
MilterConstants#SMFIR_DISCARD, MilterConstants#SMFIR_ACCEPT, MilterConstants#SMFIR_TEMPFAIL,
MilterConstants#SMFIR_REPLYCODE, unless you really, really know what you're doing.